On Imam Reza (A.S.) arrival in Marw(Khorasan),the seat of al-Ma'mun's caliphate, the Abbasid caliph showed him hospitality and great respect, then he said to him: "I want to get rid of myself of the caliphate and vest the office in you." But Imam Reza(A.S.) refused his offer. Then al-Ma'mun repeated his offer in a letter saying: "If you refuse what I have offered you, then you must accept being the heir after me." But again Imam Reza(A.S.) refused his offer vigorously.
Al-Ma'mun summoned him. He was alone with al-Fadl ibn Sahl, the Iranian noble in charge of two offices (i.e., military and civil). There was no one else in their gathering. al-Ma'mun said to Imam Reza(A.S.), "I thought it appropriate to invest authority over the Muslims in you and to relieve myself of the responsibility by giving it to you." When again Imam Reza(A.S.) refused to accept his offer, al-Ma'mun spoke to him as if threatening him for his refusal. In his speech he said, "Umar ibn al-Kahttab made a committee of consultation (shura) (to appoint a successor). Among them was your forefather, the Commander of the faithful, 'Ali ibn Abi Talib. (Umar) stipulated that any of them who opposed the decision should be executed. So there is no escape for you from accepting what I want from you. I will ignore your rejection of it."
In reply, Imam Reza (A.S.) said: "I will agree to what you want of me as far as succession is concerned on condition that I do not command, nor order, nor give legal decisions, nor judge, nor appoint, nor dismiss, nor change anything from how it is at present." al-Ma'mun accepted all of that.
On the day when al-Ma'mun ordered to make the pledge of allegiance to Imam Reza (A.S.) one of the close associates of Imam Reza (A.S.), who was present, narrates, "On that day I was in front of him. He looked at me while I was feeling happy about what had happened. He signaled me to come closer. I went closer to him and he said so that no one else could hear, 'Do not occupy your heart with this matter and do not be happy about it. It is something which will not be achieved.' "