Written to people describing what took place between him and people of Siffin
The whole thing began thus that we and the Syrians met in an encounter although we believe in one and the same Allah and the same Prophet, and our message in Islam is the same. We did not want them to add anything in the belief in Allah or in acknowledging His Messenger (pbuh)nor did they want us to add any such thing. In fact, there was complete unity except that we differed on the question of Uthman's blood while we were uninvolved in it. We suggested to them to appease the situation by calming the temporary irritationand pacifying the people till matters settled down and stabilized when we would gain strength to put matters right.
They however said they would settle it by war. Thus, they refused our offer and consequently war spread its wings and came to stay. Its flames rose and became strong. When the war had bitten us as well as them and pierced its talons into us as well as them, they accepted what we had proposed to them. So, we agreed to what they suggested and hastened to meet their request. In this way, the plea became clear to them and no excuse was left to them. Now, whoever among them adheres to this will be saved by Allah from ruin, and whoever shows obstinacy and insistence (on wrong) is the reverser whose heart has been blinded by Allah and evils will encircle his head.