May Allah forbid that you be in charge of people's affairs...
May Allah forbid that you be in charge of people's affairs after my assuming authority as Caliph, or that I issue an edict or document granting you authority over any one of them. Therefore, from now onwards guard yourself and be watchful, because if you recalcitrate till the people of Allah (are forced to) rush upon you, then matters will be closed for you and whatever can be accepted from you today will not be accepted then; and that is an end to the matter.
In reply, Amir al-mu'minin wrote this letter in which he has tried to draw Mu`awiyah's attention to the clear facts about this very battle of the Kharijites, because this battle took place in accordance with the prophecy of the Prophet while Amir al-mu'minin himself too had said before the battle that besides the people of Jamal and Siffin he had to fight against one more group and they were the "deviators" from the religion, namely the Kharijites.
The occurrence of this battle and the killing of the man with breasts (Dhu'th- thudayyah) was a clear proof of Amir al-mu'minin being in the right. If Mu'awiyah had not been obsessed with self-advertisement and lust for conquests, and had not shut his eyes against the right like his forefathers Abu Sufyan and his brother `Utbah he would have seen right and come on its path.
But compelled by his natural inclination he always evaded right and truth and kept himself blind to those sayings of the Prophet which threw light on Amir al-mu'minin's Imamate and vicegerency.
Because of being with the Prophet in the farewell pilgrimage the Prophet's saying: "Of whomsoever I am the master, `Ali is his master" was not hidden from him, and neither was the Prophet's saying that: "O' `Ali you are to me as Harun was to Musa", because of his presence on the occasion of the battle of Tabuk. In spite of all this, he passed his life in concealing right and encouraging wrong.
This was not due to any misunderstanding but it was his lust for power that kept prompting him to suppress and trample truth and justice.