Al-Mamoon Was the One Who Suggested It
What appears to us after observing the general political circumstances and from discerning al-Mamoon's political awareness which was adulterated with both caution and precision, that al-Mamoon was the one who came out with the idea of the regency as the above quoted narratives indicate. Al-Fadl ibn Sahl tried to dissuade him from doing so when he was magnifying for him their consequences, but he finally had to yield upon facing al-Mamoon's insistence.

Al-Fadl Could Not Have Suggested It
It is far-fetched to suggest that al-Fadl ibn Sahl was the one who came with the idea especially since he was a lackey and a recipient of the cash of the Barmakis and of their followers' who were all very well known for their open deviation from the line of the Alawides; so, how could it be possible that he would recommend to al-Mamoon to choose Reza(A.S.) as his successor except if the general political atmosphere of the caliphate necessitated that it should bypass the sectarian issues?
Ibn al-Athir goes beyond this in his Tarikh to suggest that al-Fadl was actually Shi'a and that he was definitely the one who suggested to al-Mamoon to choose Ali ibn Mousa Reza(A.S.) as his successor.109 He may even have narrated this in some of his narratives; for example, Abu Ali al-Hassan ibn Ahmed al-Salami writes in his book Tarikh Khurasan (history of Khurasan) saying: "Al-Fadl ibn Sahl suggested to al-Mamoon to name Ali ibn Mousa Reza(A.S.) as his regent110, and this view is adopted by a group of historians." Al-Salami also narrates saying that al-Fadl ibn Sahl, when one day al-Mamoon was discussing how he successfully transferred the caliphate with some of his very close friends (among whom al-Fadl was present), he wondered, "How would you compare my action in this regard to that of Abu Muslim [al-Khurasani]?" One of them answered, "Abu Muslim transferred caliphate from one tribe to another, whereas you have transferred it from one brother (al-Amin) to another, and there is a difference between the two cases of which you are aware." Al-Fadl said, "If it were up to me, I would rather transfer it from one tribe to another," and he suggested to him to name Ali ibn Mousa Reza(A.S.) as his regent, so he secured the oath of allegiance for him and thus dropped that of al-Mutamin Billah.111
Abdullah ibn Tahir is quoted saying: "Al-Fadl ibn Sahl suggested to al-Mamoon to seek nearness to the Almighty God and to the kin of His Messenger (S.A.W.) by naming Ali ibn Mousa (as successor) in order to wipe out the harm they had received at the hands of al-Rashid, and he could not easily reject a suggestion he made; therefore, he dispatched from Khurasan Rajaa ibn Abul Dhahhak and Yasir the servant and ordered them to seek the company of Muhammad ibn Ja'fer and Ali ibn Mousa ibn Ja'fer (and go to al-Rida, A.S.), and that was in the year 200 A.H."112

Al-Mamoon Asserts the Idea Was His
In contrast with the above, al-Rayyan ibn al-Salt says: "There was a huge multitude of army leaders and civilian dignitaries as well as huge crowds of commoners assembled to witness the nomination of Reza(A.S.), and they were saying that that was due to the arrangement of al-Fadl Dhul-Riyasatayn, and al-Mamoon came to know about it, so he sent for me in the midst of the night, and I stood before him. He said, `O Rayyan! It has come to our knowledge that people say that the nomination of Reza(A.S.) was the arrangement of al-Fadl ibn Sahl Dhul-Riyasatayn.' I said, `O commander of the faithful! They indeed say so.' He said, `Fie upon you, O Rayyan! How dare anyone come to the caliph, after his subjects are completely loyal to him and so are his leaders and he is well-seated in his post, and tells him to hand over the caliphate to someone else? Is this conceivable?' I said, `By God, no, O commander of the faithful! Nobody dares to do that.' He said, `No, by God! What they say is not true, but I shall tell you of the reason for that.' Then he mentioned the reason to be his pledge to God that if he regained his post and became in charge, he would conduct regency the way God wanted it to be."113

Historical Analysis of Its Attribution to al-Fadl
If we observe the Imam's conduct towards al-Fadl, his view about the Imam (A.S.), and his warning to al-Mamoon against giving him the reins of leadership of his government, we cannot help endorsing the view which says that the choice of regent was something al-Mamoon thought about and politically calculated. Al-Fadl was not ignorant of the status of the Imam (A.S.) and the power of his influence should he side with al-Mamoon, and he was not naive to the extent that he would jeopardize his powerful influence by getting involved in al-Mamoon's apparatus in a confrontation with a more powerful influence.
As regarding what others have stated that it was he who suggested to al-Mamoon to do so, this may be attributed to the fact that if any event happened to the ruling apparatus, its credit was often given to the person with the strongest influence in that apparatus, the one who was strong enough to face the public opinion bearing full responsibility for any action taken by the government. Abul-Fadl, according to public opinion of the time, enjoyed the widest influence and the strongest word with the caliph al-Mamoon, and when al-Mamoon was about to make a decision regarding the appointment of the Imam (A.S.) as his successor, people would think that al-Fadl must have been inspired the idea. It was held that he must have been the one who subjected al-Mamoon to his views in all his political measures, enforcing a complete control over them. For this reason, we see that when the letter of al-Hassan ibn Sahl reached Isa ibn Muhammad ibn Khalid, in which he was informing him that al-Mamoon was getting ready to nominate Reza(A.S.) as his successor and that he had already ordered him to remove the official black government uniforms and substitute them with green ones and ordered the army and its commanders as well as Banu Hashim to swear the oath of allegiance to him as such and to require all the residents of Baghdad to do likewise, some people said they would swear allegiance but would not wear green while others said that they would do neither and that they would not let the descendants of the Abbasides lose their grip over the government, adding that it was a "conspiracy" inspired by al-Fadl ibn Sahl.114
The accusation of the people of Baghdad of al-Fadl was inspired only because of his having the strongest influence over the government, and we think it is not too far to believe that the publicity of the rumour that the idea was suggested to al-Mamoon by al-Fadl was actually the doing of al-Fadl himself in order to safeguard his own status in public opinion since he did, indeed, have the strongest influence over the caliph's actions. When al-Mamoon asks one of his close friends about his own opinion regarding the comparison between what he did and what Abu Muslim had done, he brags about transferring the caliphate from one tribe to another just as Abu Muslim had done in order to boast to his listeners of having the ability to do with the caliphate whatever he pleased, and that the arrangement of the regency issue was done according to his own instructions rather than those of anyone else.

Al-Fadl Sows Mischief Between the Imam and al-Mamoon
Al-Fadl actually did try to transfer the caliphate from one tribe to another in order to satisfy by so doing his own personal conceit and in pursuit of his own personal ambition to be a second Abu Muslim, so he enters the residence of Reza(A.S.) accompanied by Hisham ibn Amr to say: "O son of the Messenger of God! I have come to you to speak in private; so, please clear the place," then al-Fadl brings out of his pocket an oath sworn by the sanctity of emancipation, divorce, and whatever has no kaffara, and both men say to him, "We have come to you to say a word of truth and honesty, and we know that your word is most effective, and the right belongs to you. O son of the Messenger of God! What we say with our tongues is attested to by our own conscience; otherwise, we would emancipate all that we have, and all our women are henceforth divorced, and I shall be required to perform the pilgrimage thirty times on foot... that we shall kill al-Mamoon and put you in charge so that right goes back to you," but he did not listen to them but cursed them and said, "You both have proven ungrateful to the blessings God has blessed you with; therefore, you will not be safe from what you have said, and I shall not get what you promise even if I were to agree to what you say." When al-Fadl and Hisham heard the Imam (A.S.) say so, they realized that they were mistaken in their calculations; therefore, they went back to al-Mamoon after telling Reza(A.S.) that they were only testing him. Before leaving, they were told by the Imam (A.S.), "You have lied, for your hearts certainly relish what you have just said to me, but you found me not exactly as you had hoped." When they entered al-Mamoon's court, they said: "O commander of the faithful! We have just visited Reza(A.S.) and we tested him and wanted to sift his mind about you, so we said what we said and he said too," whereupon al-Mamoon said, "You have done well." So when they came out, Reza(A.S.) went to see him and they remained by themselves and Reza(A.S.) informed him of what they both had said and enjoined him to protect himself from their mischief. When al-Mamoon heard that from Reza(A.S.), he knew that the Imam (A.S.), not those two men, was the truthful.

Subjective Analysis of the Dialogue
Should this story be true, it would be a proof showing us the precise political dimension of a dangerous move whereby al-Fadl tried to score a victory for himself and strengthen his own position which was being weakened by his being distanced from the power nucleus after the nomination of Imam Reza(A.S.) as the successor. Al-Fadl, by so doing, was either serious in his offer or a pretender.
If we suppose that he was serious, what would then his objective be? What we can understand as an interpretation of the situation is that al-Fadl was trying by so doing to involve the Imam (A.S.) in the plotting of a conspiracy to assassinate al-Mamoon, and when caliphate was to be transferred to the Imam (A.S.), since he was the heir to the throne, al-Fadl would be in a position to hold the reins of government and enforce his control over its authority, making the Imam's participation in the plot as a blackmail against the Imam (A.S.) whereby he could threaten him should he try in any way to restrict his influence. Or, after eliminating al-Mamoon, it would be easy for him to eliminate Imam Reza(A.S.) as well through a little help from groups opposing the Imam's caliphate.
Had al-Fadl been truly honest in his offer to transfer the caliphate to the Alawide dynasty and to eliminate al-Mamoon, and had he been confident of the practicality of the idea, what made it necessary for him to consult the Imam (A.S.) regarding all of that since caliphate would be transferred to the Imam (A.S.) automatically and without any obstacle after the elimination of al-Mamoon since he was the appointed successor?
If we suppose that he was a pretender in his offer, as al-Fadl tried to assert after the Imam's rejection of his plot, then the goal he was trying to achieve becomes quite clear, for he would then desire to disturb the standing relationship between the Imam (A.S.) and al-Mamoon and, at the same time, prove to al-Mamoon his loyalty to his government and concern about its security.

Imam Foils al-Fadl's Attempt
The Imam (A.S.) foiled his attempt to achieve his goal in either possibility, and Imam al-Rida's assertion to al-Mamoon that al-Fadl was quite serious about his offer is a strong reason added to the other reasons which caused al-Mamoon to eliminate al-Fadl at a later time. Having absorbed all the above, we can be easily satisfied that the regency concept was due to al-Mamoon's conviction of the persisting need for it in order to achieve some political gains the government was concerned about achieving.
We can also be satisfied that the publicity al-Fadl ibn Sahl awarded that arrangement cannot be proven even when many historians insist it could, for attributing Shi'aism to him was due to the rumours which said that al-Fadl was the one who offered al-Mamoon the most encouragement to name Reza(A.S.) as his successor, but we opt for the opposite due to the lack of evidence after having analyzed all situations as stated above.

Al-Mamoon Calls Imam to Him
Having evaluated the general status of the political policies of his government, which were surrounded with tumultuous events starting with Baghdad going back against its promise of support to him and passing by the Shi'a Alawide throngs surrounding his base of government in Khurasan and ending with the Alawide rebellions in Iraq, Hijaz and Yemen, al-Mamoon thought of curing this weak point by a brilliant acceptable political move which would be something to divert the attention of the Alawides and the Shi'a residents of Khurasan and, at the same time, a terrible threat to the Abbaside throngs in Baghdad that would guarantee influence for his position and control over all parties, and this could not be achieved without naming Imam Ali ibn Mousa Reza(A.S.) as the successor to the throne. And so it happened; he sent letters to the Imam (A.S.) ordering him to go to Marw. The Imam (A.S.) refused, and a great deal of correspondence ensured between both men till al-Mamoon convinced him finally and through his own special ways to go there, sending him a special force to escort him on his way which included al-Dhahhak, or, according to al-Mufid and Abul Faraj al-Asbahani, al-Jalloudi. History books do not say much about that trip except small bits and pieces which do not provide us with a clear vision of its nature and mission.

Al-Mamoon Determines Imam's Route
Al-Mamoon had already ordered his messenger to take a group of dignitaries who were descendants of Abu Talib to the Basrah highway, then to al-Ahwaz and Persia, keeping in mind that the alternate route, which was Kufa-al-Jabal-Kerman Shah-Qum, was mostly inhabited by Shi'as and it has their strongholds, and they might be carried away by their enthusiasm upon finding out that the Imam (A.S.) was among them and might decide to keep him there and thus involve the government in dangerous consequences which might cause its weakening and collapse.

Imam in Nishapur
When he entered Nishapur, he stayed at a neighbourhood called al-Qazwini where there were crowds of pigeons, the pigeons which they call today Reza(A.S.) pigeons, and there was a spring there the water of which had receded, so he hired workers who repaired it till its water became plentiful. He had a pool built on its outside where stairs were also built according to his instructions leading to the low level of the spring water, so the Imam (A.S.) went down, made his ablution, came out and said his prayers on the outside.

Story of the Gold Chain
According to Tarikh Nishapur, as quoted in Al-Fusool al-Muhimma by Ibn al-Sabbagh the Malekite, when the Imam (A.S.) entered Nishapur on his way to Marw, he was inside a dome with curtains conveyed on a gray mule, and he went through Nishapur where the two Imams who memorized the ahadith of the Prophet (S.A.W.) and the students of the Sunnah of the Prophet (S.A.W.), namely Abu Zar'a al-Razi and Muhammad ibn Aslam al-Toosi, with countless scholars and seekers of knowledge, traditionists and critics, and they both approached the Imam (A.S.) saying, "O most honourable dignitary and the son of the master Imams! By the rights of your purified forefathers (A.S.) and your glorious ancestors, could you please let us see your blessed face, and could you narrate for us hadith from your forefathers quoting your grandfather Muhammad (S.A.W.) whereby we can remember you?" So he ordered to have the mule halted, and he cooled the eyes of the throngs with his blessed sight. He had two locks of hair on his shoulders, and people from all classes were standing and looking at him, some loudly crying and rolling in the dust before him while others were kissing the hooves of his mule. The noise became much louder, and the leading scholars loudly called upon people, "O folks! Listen and learn! Listen to what benefits you and do not harm us by your loud screams and cries!" The person who requested permission to write down then was Abu Zar'a Muhammad ibn Aslam al-Toosi. Ali ibn Mousa Reza(A.S.) said: "My father Mousa al-Kazim (A.S.) narrated to me from his father Ja'fer al-Sadiq (A.S.) from his father Muhammad al-Baqir (A.S.) from his father Ali Zaynul-Abidin (A.S.) from his father, the Martyr of Kerbala (A.S.), from his father Ali ibn Abu Talib (A.S.) saying: `My loved one, and the pleasure of my eyes, the Messenger of God (S.A.W.), narrated to me once that Jibrail (Gabriel) told him that he had heard the Lord of the Throne, Glorified and Praised be His Name, saying, `The kalima of LA ILAHA ILLA-ALLAH is My citadel; whoever said it would enter My citadel, and whoever entered My citadel was safe from My retribution.'" Then he let the curtains loose on the dome and went away while the scribes outnumbered twenty thousand.
Abu Na'im said in Hilyat al-Awliya, after quoting the narrative above, "This is a firm hadith famous in this way of narration through the line of narrators from among the Purified Ones (A.S.) who quote their forefathers, and some of our predecessors who were traditionists used to say whenever this tradition was narrated that if this narrative was narrated to a madman, he would come back to his senses."