I asked Abu Waail, "Were you present at Siffin?"He replied, "Yes, and I heard Sahl bin Hunayf say, 'People, hold your own opinions in suspicion when it comes to your religion. 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. We never placed our swords upon our shoulders facing a matter that alarmed us except that they led us to an outcome we recognized, save for this present affair.'"Abu Waail said, "I witnessed Siffin, and what a wretched Siffin it was."
سألت أبا وائل: هل شهدت صفين؟ قال: نعم، فسمعت سهل بن حنيف يقول: يا أيها الناس اتهموا رأيكم على دينكم لقد رأيتني يوم أبي جندل، ولو أستطيع أن أرد أمر رسول الله ﷺ لرددته، وما وضعنا سيوفنا على عواتقنا إلى أمر يفظعنا إلا أسهلن بنا إلى أمر نعرفه غير هذا الأمر. قال: وقال أبو وائل: شهدت صفين، وبئست صفون.
Bukhari related it in the Book of Holding Fast to the Quran and Sunnah (7308), and Muslim related it in the Book of Jihad and Expeditions (1785: 95), through various routes from Amash, and the wording given here is Bukhari's.It is also related from Uzai that he said, "Umar bin Abdul-Aziz wrote that no one has any personal opinion regarding the Book of Allah, rather personal opinion applies only to matters on which no scripture has been revealed and no sunnah has been established by the Messenger of Allah ﷺ. No one has any personal opinion regarding a sunnah that the Messenger of Allah ﷺ established."Darimi related it (446) with a sahih chain of transmission.