Musnad Ahmad 5:273

'Abdullah narrated to us, Abu Bakr ibn Abi Shaybah narrated to us, Sharik narrated to us, from Abu Ishaq, from Abu Juhayfah, who said: 'Ali said: "The best of this nation after its Prophet is Abu Bakr, and after Abu Bakr, 'Umar. And if I wished to inform you of the third, I would have done so."