Sahih Bukhari Volume 5, Book 57, Hadith Number 20.

Narated By Muhammad bin Al-Hanafiya : I asked my father (‘Ali bin Abi Talib), “Who are the best people after Allah’s Apostle ?” He said, “Abu Bakr.” I asked, “Who then?” He said, “Then ‘Umar. ” I was afraid he would say “Uthman, so I said, “Then you?” He said, “I am only an ordinary person.