Sahih Bukhari Volume 9, Book 92, Hadith Number 411.

Narrated By Al-A’mash : I asked Abu Wail, “Did you witness the battle of Siffin between ‘Ali and Muawiya?” He said, “Yes,” and added, “Then I heard Sahl bin Hunaif saying, ‘O people! Blame your personal opinions in your religion. No doubt, I remember myself on the day of Abi Jandal; if I had the power to refuse the order of Allah’s Apostle, I would have refused it.

We have never put our swords on our shoulders to get involved in a situation that might have been horrible for us, but those swords brought us to victory and peace, except this present situation.’ ” Abu Wail said, “I witnessed the battle of Siffin, and how nasty Siffin was!”