Musannaf Ibn Abi Shayba 38:28

Ibn Idris narrated to us, from Hassan bin Furat, from his father, from Umayr bin Sa’d, who said: When Ali returned from the Battle of the Camel and prepared for Siffin, the [tribe of] Nakha’ gathered until they entered upon al-Ashtar. He said, "Is there anyone in the house except a Nakha’i?" They said, "No." He said, "Indeed, this nation intended [to harm] its best [person] and killed him. We marched to the people of Basra, a people upon whom we had a pledge of allegiance, and we were victorious over them due to their breaking of it. And you are marching tomorrow to the people of Syria, a people upon whom you have no pledge of allegiance. So let a man among you look to where he places his sword."