Musnad Ahmad 5:270

Abdullah narrated to us, Salih ibn Abdullah al-Tirmidhi narrated to me, Hammad narrated to us, from Asim. And Ubaydullah al-Qawariri narrated to us, Hammad narrated to us—al-Qawariri said in his narration—he said: Asim ibn Abi al-Najud narrated to us, from Zirr—meaning ibn Hubaysh—from Abu Juhayfah, he said: I heard Ali saying: "Shall I not inform you of the best of this nation after its Prophet? Abu Bakr." Then he said: "Shall I not inform you of the best of this nation after Abu Bakr? Umar."