Muhammad bin Sirin (may Allah have mercy on him) said: I do not know of anyone who disliked fighting robbers and the Haruriyyah, except a cowardly man. The Shaykh said: A group disliked this fighting, and their evidence is that many Companions kept away from the Battle of Siffin; among them were Sa'd bin Abi Waqqas, Usamah bin Zayd, Muhammad bin Maslamah (may Allah be pleased with them) and others. It is reported from some of them that they said: "My opinion was wrong." Some said: "He killed a Muslim, thinking that he was seeking refuge through his Islam." Then he made a covenant with Allah Almighty that he would never again kill anyone who says 'La ilaha illallah' (There is none worthy of worship except Allah). Some considered fighting during division to be grave and regarded it as 'fighting in division', and some wished that someone other than him should take charge of it. Most of them held that Sayyiduna Ali (may Allah be pleased with him) was in the right in this fighting. His fighting was against those who turned away from obedience to him, namely the people of Syria, who refused to obey the imam; and he fought the people of Basrah because they thought that he himself had killed Sayyiduna Uthman (may Allah be pleased with him), or at least had taken part in his killing. For this reason they fought him, or because they found fault with his imamate. They took as evidence of the rebellion and opposition of the people of Syria the consultative council (shura) formed by the Commander of the Faithful, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him), and also that the remaining members of the shura had pledged allegiance to him before the division, and that by virtue of his distinctive qualities he was the most deserving of the imamate; and that people had recognised in his opponents the sign that the Messenger of Allah (peace and blessings be upon him) had indicated.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16785