After Imam al-Ridha (a.s.) arrived in Marv in 201/816 at the insistence of Ma’mun and his frequent letters of invitations to him and his rejecting the excuses that the Imam made, Ma’mun first offered him the caliphate and told him: “O son of the Apostle of Allah! I am aware of your excellence, erudition, asceticism, piety, and devotedness and consider you as more deserving of the caliphate than myself.”
Imam al-Ridha (a.s.) said, “I take pride in my devotion and servitude to Allah, hoping to be secure from the torment of the Hereafter by asceticism in this world, to gain profits by minding the unlawful, and to achieve high standing before Allah through humility in this world.” Ma’mun said, “I am of the opinion to resign from caliphate and leave it to you and swear allegiance to you!”
Imam al-Ridha (a.s.) said, “If this caliphate is rightfully yours and Allah has assigned it to you, it is not permissible for you to take off the garment that Allah has clothed you with and give it to another one; and if you have no rights in caliphate, you are not permissible to give me what is not yours.”
Ma’mun said, “O son of the Apostle of Allah! You have to accept this task.” His holiness said, “I never do this voluntarily.”
For a long time Ma’mun kept on insisting until he got disappointed in his holiness and said to him: “If you do not accept caliphate and do not wish me to swear allegiance to you, then be my heir apparent so that you become the caliph after my death.”
Imam al-Ridha (a.s.) said, “By God, my father heard from his forefathers who heard from Commander of the Faithful that the Apostle of Allah (S) said that I would depart from the world before you do, while I will be tyrannically killed with poison and the angels of the heaven will cry over me, and I will be buried in a strange land near Harun’s tomb.”
Ma’mun cried and said, “O son of the Apostle of Allah! Who would kill you or would be able to harm you so long as I am alive?”
His holiness said, “I would say who will kill me if I wished to.”
Ma’mun said, “O son of the Apostle of Allah! By saying this, you intend to lighten burden and take it off your shoulders so that the people say that you are an ascetic in the world!”
Imam al-Ridha (a.s.) said, “By God, since my Almighty and Glorious Lord created me, I have not told a lie, and I have never abstained from the world for the sake of [profiting from] the world; and I know what your intention is.”
Ma’mun said, “What is my intention?” His holiness said, “Am I really safe?” Ma’mun said, “You are safe.” The Imam said, “You want the people to say that ‘Ali b. Musa al-Ridha was not uninterested in the world; rather, it was the world that was uninterested in him; do you not see that he accepted heir apparency to assume the caliphate.”
At this moment Ma’mun became furious and said, “You always make me encounter with what disturbs me and remain safe from my anger.
I swear by God that you will accept the heir apparency or I will force you to, otherwise, I will cut your head off.” Imam al-Ridha (a.s.) said, “The Almighty and Glorious Allah has forbidden me from putting myself to death with my own hands. If it is so, do whatever you like to do; I will accept, provided that I would –neither install nor dismiss anyone, nor change any rules or procedures, and just be an advisor from a distance.”
Ma’mun accepted these conditions and appointed his holiness as his heir apparent.1
It is related in some traditions that the Imam said, “O God! You have prohibited me from putting myself to death; if I do not accept his heir apparency, I will be few steps away from death [i.e., to be killed] by him; I am compelled and distressed… O God! There is no pledge of allegiance except to You; there is no guardianship except from You; help me succeed in establishing Your religion and reviving the Sunna of Your Prophet; You are the Mawla and Helper, and the best Mawla and Helper as you are.” Then, while weeping and sorrowful, he accepted the heir apparency with the above-mentioned conditions.2
After Imam al-Ridha (a.s.) accepted the heir apparency in this manner, Ma’mun ordered that his son ʿAbbas, his commanders, judges, servants, and children all come and swear allegiance to the Imam.
He spent a lot of properties to content the commanders; all agreed except three of them called Jalludi, ‘Ali b. ‘Imran, and Abu Musa (or Abu Yunus) who did not swear allegiance to the Imam al-Ridha (a.s.) and Ma’mun sent them to prison.
The allegiance ceremony was held most splendidly, coins were minted in the name of his holiness, and sermons were delivered on the pulpits.