From Sahih Bukhari (Hadith 3181):
Narrated Al-A`mash:I asked Abu Wail, "Did you take part in the battle of Siffin?" He said, 'Yes, and I heard Sahl bin Hunaif (when he was blamed for lack of zeal for fighting) saying, "You'd better blame your wrong opinions. I wish you had seen me on the day of Abu Jandal. If I had the courage to disobey the Prophet's orders, I would have done so. We had kept out swords on our necks and shoulders, for a thing which frightened us. And we did so, we found it easier for us, except in the case of the above battle (of ours)
Reference: Bukhari 3181