Sahih Bukhari Volume 5, Book 57, Hadith Number 56.

Narated By Ubaida : Ali said (to the people of ‘Iraq), “Judge as you used to judge, for I hate differences (and I do my best) till the people unite as one group, or I die as my companions have died.” And narrated Sad that the Prophet said to ‘Ali, “Will you not be pleased from this that you are to me like Aaron was to Moses?”