They even prevailed upon Mamun to notify the appointment of his brother...

They even prevailed upon Mamun to notify the appointment of his brother, Mutaman, as heir apparent to the throne. He was later acknowledged as the caliph and became known as Mutasim Billah. Besides, during the tenure of Imam Reza’s (AS) heir-ship the special mark of Bani Abbas, the black attire was replaced by the green, which was gaining general currency all around.

This practice was abrogated and the wearing of black clothes was made compulsory so that the old traditions of Bani Abbas could be preserved. All this had convinced the Abbasids that they now had complete hold over Mamun. Now they had complete hold over Mamun. Now Mamun’s intention to make Imam Muhammad Taqi (AS) his son in-law again became a cause of worry for them.

They were so upset that that they could not restrain their feelings and in the form of a deputation they waited on Mamun and gave vent to their sentiments. They told him plainly that they did not relish his very manner of dealing with Imam Reza (AS) However, at least with respect to his age, his attributes and accomplishments, he could be considered as worthy of honor and esteem. However, Muhammad, his son was only a child of quite a tender age.

To give preference to a mere child over great scholars and respectable personalities and to pay him so much respect did not at all behoove the caliph. They further asked what benefit had they gained from the marriage of Umme Habiba with Imam Reza (AS) that Ummul Fazl was now being given in wedlock to Imam Muhammad Taqi (AS)?

The reply of Mamun to their protests was: “No doubt Muhammad is of tender age, but I have fully assessed his capabilities and have found that he is the perfect successor of his father with respect to his attributes and accomplishments. These august scholars of the Islamic world whom you are referring to cannot compete with him in the field of knowledge and learning.

You can test his capabilities if you like but then only perfectly reasonable but was in fact a sort of a challenge that obliged the deputationists to agree to a competition. Amongst the Abbasid rulers, Mamun occupied a special status and the historians have recorded that he was himself reckoned amongst great jurists. For that reason, his own judgement in this respect did not carry any lesser weight.

Nevertheless, they were not satisfied with that and selected the greatest scholar of Baghdad Yahya bin Aktham, to argue with Imam Muhammad Taqi.