From Sahih Bukhari (Hadith 4189):
Narrated Abu Wail:When Sahl bin Hunaif returned from (the battle of) Siffin, we went to ask him (as to why he had come back). He replied, "(You should not consider me a coward) but blame your opinions. I saw myself on the day of Abu Jandal (inclined to fight), and if I had the power of refusing the order of Allah's Apostle then, I would have refused it (and fought the infidels bravely). Allah and His Apostle know (what is convenient) better. Whenever we put our swords on our shoulders for any matter that terrified us, our swords led us to an easy agreeable solution before the present situation (of disagreement and dispute between the Muslims). When we mend the breach in one side, it opened in another, and we do not know what to do about it
Reference: Bukhari 4189