Sahl and his entrusting all his affairs to him...

Sahl and his entrusting all his affairs to him, while he was originally from Persia. Moreover, his brother al-Amin hated him, intended to do evil deeds toward him, and schemed against him, for he competed with him for the authority. Accordingly, al-Mamūn wanted to reinforce his position, to strengthen his influence, and to overcome those who harbored malice against him. As a result, he vested the office in Imām Ali b.

Mūsā al-Ridā, peace be on him, the greatest personality in Islamic world and son of Imām al-Sādiq, who was the first to supply Islamic world with thoughts and knowledge. Moreover large part of the Muslims believed in his Imāmate and showed friendship to him. For this reason al-Mamūn took the initiative and designated him as his successor over this important office in Islamic world.

Al-Mamūn undertook the leadership of Islamic state while he was fully aware of that Islamic society harbored hate and detest against the Abbāsid family, who oppressed the Muslims, appropriated their affairs, wreaked all kinds of oppression and tyranny upon the Alawides, the summoners to social justice. So the Muslims wished for the return of the Umayyad reign though it was famous for cruelty and torture.

In this connection, the poet says: Would that the tyranny of the Marwānis returned to us, and would that the just of the Abbāsids was in the fire. Another poet says: I do not think that tyranny will come to an end while over the community is an Abbāsid governor. Al-Mamūn intended to open a new page for the citizens and to conceal the policy of his fore-fathers, so he appointed Imām al-Ridā the hope of Islamic community, peace be on him as a successor after him.

Most al-Mamūns Army, officers and soldiers, were from among the Shiites who adopted the Imāmate of Imām al-Ridā, peace be on him, so he wanted to win their affection and loyalty. Surely the revolt against the Abbāsid government broke out and extended to most Islamic regions, and the motto of the revolutionaries was: The summons to al-Ridā from among the family of Mohammed, may Allah bless him and his family. The revolutionists responded to this allegiance al-Mamūn pledged to the Imām.

In the meantime, he gave the Imām the nickname of al-Ridā (i.e. the consent), that, through that, he might attract the feelings of the revolutionaries, and they pledged allegiance to him, and he got rid of the danger which encircled his state and was about to wrap up its banner and fold up its principal features.