Musnad Ahmad 5:467

'Abdullah narrated to us, Abu Bahr 'Abd al-Wahid al-Basri narrated to me, Abu 'Awanah narrated to us, from Khalid ibn 'Alqamah, from 'Abd Khayr, who said: 'Ali said, when he had finished with the people of Basra: "The best of this nation after its Prophet, may Allah bless him and grant him peace, is Abu Bakr, and after Abu Bakr is 'Umar. And events have occurred among us, regarding which Allah will do as He wills."