Musnad Ahmad 5:345

Abdullah narrated to us, Nasr ibn Ali al-Azdi narrated to me, Bishr ibn al-Mufaddal narrated to us, from Shu'bah, from Habib ibn Abi Thabit, from 'Abd Khayr: I heard Ali saying: "Shall I not inform you of the best of this nation after the Messenger of Allah, may Allah bless him and grant him peace? Abu Bakr, and Umar."