Mamun invited Imam Ridha’ (a.s) to his state capital and insisted upon him to accept his heir apparency. The Imam (a.s) continued to refuse it and told him in unequivocal terms that he would never be able to become his heir. He said that he would be poisoned to death before Mamun died. Mamun said, “Who can dare kill you, as long as I live?” The Imam said, “If it had not be expedient I would have even told you the name of my killer.” When Mamun realized that he was not relenting he was infuriated and he said, “By this refusal you aim to become famous for your piety and abstemiousness and that my helpless and weakness may become obvious.” Imam said, “I have never lied in my life. It does not befit me to pretend austerity for the sake of material gains. But your insistence in this matter shows that you want to prove to the world that Ali Ibn Musa (a.s) was not abstemious in the real sense. Material wealth had itself deserted him for some time. And when it came back to him he became involved in it with all desire and fondness.”
Hearing this reply Mamun became more angry and making a show of his apparent strength and power said, “If you do not accept my heir-apparency and continue to refuse it, I would have you killed.” Imam (a.s) said, “If the matter has reached to this point, I will accept it with the conditions that I will not be involved in administration, I would not refrain from enjoining good and forbidding evil and I will never support you in an unlawful matter.”
It is mentioned in Uyoon Akhbar ar-Ridha’ that when Mamun held the commemorative function to celebrate the appointment of Imam Ridha’ (a.s) as the heir apparent, he requested the Holy Imam (a.s) to deliver a sermon. The Imam came to the pulpit and after invoking praise on the Almighty and glorifications of the Holy Prophet (S) he said,
“People! Due to our relationship with the Messenger of Allah! We have a right upon you and in the same way you have a right upon us. When you have fulfilled our rights it is also necessary on us to observe your rights. Thanks be to Allah that He has protected those of our rights that people had destroyed and raise those of our affairs that people had demolished. For a period of eighty years the people of disbelief and disobedience used to invoke curse upon us from the pulpits and they continued to hide our excellences and made false allegations against us. But Allah intended that out fame should rise high.
O People! I did not accept the heir-apparency because I desire post and status and I am desirous of rulership. I accepted it so that if I see you walking on the wrong path I may restrain you, whether you listen to me or not. I inform you that I will never hesitate in the expression of truth, even though I may be killed for this. The existence of us, Ahl ul-Bayt in the world is only for the purpose that we may fearlessly support truth.”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams