Musannaf Ibn Abi Shayba 38:100

Al-Fadl ibn Dukayn narrated to us, from Hasan ibn Salih, from Abdullah ibn al-Hasan, who said: I heard him say: Ali said to the two arbitrators, "On the condition that you judge by what is in the Book of Allah, and the entire Book of Allah is mine. If you do not judge by what is in the Book of Allah, then there is no arbitration for you."