Musnad Ahmad 5:317

Sufyan ibn ‘Uyaynah narrated to us, from Ibn Abi Khalid—and [in another chain] Abu Mu’awiyah narrated to us, from Isma’il, from al-Sha’bi, from Abu Juhayfah, who said: I heard ‘Ali saying: "The best of this nation after its Prophet are Abu Bakr and ‘Umar. And if I wished, I would have told you of the third."