Musnad Ahmad 5:476

Waki’ narrated to us, from Sufyan and Shu’bah, from Habib ibn Abi Thabit, from ‘Abd Khayr, from ‘Ali, that he said: "Shall I not inform you of the best of this nation after its Prophet, may Allah bless him and grant him peace? Abu Bakr, then ‘Umar."