Musannaf Ibn Abi Shayba 38:96

Muhammad ibn al-Hasan al-Asadi narrated to us, he said: Yahya ibn Muhallab narrated to me, from Sulayman ibn Mihran, he said: Someone who heard Ali on the day of Siffin narrated to me, while he was biting his lip: "If I had known that the matter would turn out like this, I would not have gone out. Go, O Abu Musa, and judge, even if my neck were to fall."