Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / 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.