Sahih Muslim Book 19, Hadith Number 4408.

It has been narrated through a different chain of transmitters on the authority of Abu Wa’il who said: I heard Sahl b. Hunaif say at Siffin: Blame (the hollowness) of your views about your religion. I thought to myself on the day of Abu Jandal that if I could turn down the order of the Messenger of Allah (may peace be upon him), I would. The situation was so difficult that if we mended it at one place, it was rent at another.