It is mentioned in the History of Tabari that one day some people gathered in a special gathering of Mamun in order to debate with the Imam regarding Imamate. They selected Yahya bin Dhahak, who was a great Sunni scholar of that time to represent the Sunni group and debate with the Imam. Imam (a.s) told Yahya to ask him whatever he liked.
Yahya: I want you to pose questions to me. Imam: All right, I will ask you. Tell me. What do you say about one who claims to be truthful himself, but lies about those who are truthful? Is such a person truthful? Tell me, with regard to religion is he on the right or falsehood.
Hearing this Yahya felt silent. After sometime Mamun demanded a reply. He said, “O Chief! I have no reply to this.” Mamun asked Imam, “Please explain to me what you asked, that a great scholar like Yahya cannot reply.” Imam said, “What reply can this poor Yahya give? If he says that the truthful ones did not lie, his reply would be incorrect. When Abu Bakr sat on the pulpit of the Prophet and confessed his helplessness and said: Though I am a ruler on you I am not better than you. Then after this his claim that he was a caliph of the prophet was incorrect or not? In the circumstances that he was not better than the people how did he become a ruler? It is necessary for the chief to be superior to the subjects. In addition to this he said from the pulpit: There is a Satan who overcomes me. Then how can he be an Imam? An Imam is one who is safe from the Satan. Thirdly, how can such a person become a ruler and caliph whose followers themselves say: The allegiance of Abu Bakr was a hasty matter. Allah saved the people from its mischief. If anyone does it again, I will kill him. When Mamun heard this, he told those present in the assembly, “All those present here may please leave. Did I not advise you against debating with him? They are the inheritors of the knowledge of the Prophet.”
Reference :Akhlaq al-A’imma, Morals & Manners of the Holy Imams