While at Siffin, he said, "People, distrust your own opinions. By Allah, I saw myself on the day of Abu Jandal, and had I been able to reject the command of the Messenger of Allah ﷺ, I would have rejected it. By Allah, we never placed our swords on our shoulders for any matter except that they led us smoothly to an outcome we recognized, except for this affair of yours."
وهو بصفين: أيها الناس اتهموا رأيكم والله لقد رأيتني يوم أبي جندل ولو أني أستطيع أن أرد أمر رسول الله ﷺ لرددته، والله ما وضعنا سيوفنا على عواتقنا إلى أمر قط، إلا أسهل بنا إلى أمر نعرفه إلا أمركم هذا.
Bukhari narrated it in the Book of jizya and Truces (3181), and Muslim in the Book of jihad and Expeditions (95:1785), both through the route of Amash, from Abu Waail (Shaqiq bin Salama), who said, "I heard Sahl bin Hunayf say," and he mentioned it.