Musannaf Ibn Abi Shayba 38:115

Abu Mu'awiyah narrated to us, from al-A'mash, from Shaqiq Abu Wa'il, who said: Sahl ibn Hunayf said on the day of Siffin: "O people, distrust your own opinion. For by Allah, we did not place our swords upon our shoulders with the Messenger of Allah (peace and blessings of Allah be upon him) for a matter that distressed us, except that it led us easily to a matter that you recognize, other than this."