After the Muslims took oath of allegiance to Imam Ali (a), he wrote the following letter to Mu'awiya. (Muhammad ibn `Umar) al-Waqidi has mentioned this in his "Kitab al-Jamal"
From the servant of Allah, `Ali Amir al-mu'minin to Mu`awiyah son of Abu Sufyan:
Now, you are aware of my excuses before you people and my shunning you till that happened which was inevitable and which could not be prevented. The stroy is long and much is to be said. What was to pass has passed and what was to come has come. Therefore, secure (my) allegiance from those who are with you and come in a deputation of your people to me; and that is an end to the matter.