Abu Wa'il relates that Sayyiduna Sahl bin Hunayf (may Allah be pleased with him) said on the day of Siffin: "O people! Hold your own opinion suspect in matters of religion. I saw us when we were with the Messenger of Allah (peace and blessings be upon him), and had we been able to turn back his decision (with our own opinion), we would have done so. We never placed our swords on our shoulders for any matter except that they made the way easy for us towards a matter that we recognised, except for this (present) matter of ours. And I saw myself on the day of Abu Jandal; had we been able to turn back the decision of the Messenger of Allah (peace and blessings be upon him), we would have done so."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5577