Al-Ma'mun thought about the state of the caliphate after him and wanted to place it in a man suitable for it in order to clear his liability - as he claimed - so he mentioned that he considered the conditions of the notables of the two houses: So he entrusted him and wrote a book in his own handwriting, and obliged al-Rida, peace be upon him, to do so, but he refused and then answered, and put his handwriting on the back of al-Ma'mun's book with the meaning: I answered in compliance with the order, although the Jafr and the Jami"ah indicate the opposite, and witnesses testified to this.
Al-Fadl ibn Sahl, al-Ma'mun's minister, was the administrator and promoter of this matter, so people pledged allegiance to Ali ibn Musa after al-Ma'mun, and he was called al-Rada from the family of Muhammad (peace be upon him).
Al Fakhri Fi al-Aadab Al-Sultaniyah
Al Tuwariyat ((٤) رجال إسناده ثقات. ) (and the inspector is a Nasibi so we accept his admission and reject his opinions)