Musnad Ahmad 5:272

Isma'il ibn Ibrahim narrated to us, Mansur ibn 'Abd al-Rahman—meaning al-Ghadani al-Ashall—informed us, from al-Sha'bi, Abu Juhayfah—whom 'Ali used to call Wahb al-Khayr—narrated to me, he said: 'Ali said: "O Abu Juhayfah, shall I not inform you of the best of this nation after its Prophet?" He said: I said: "Yes." He said: And I did not think that anyone was better than him. He said: "The best of this nation after its Prophet is Abu Bakr, and after Abu Bakr, 'Umar, and after them a third person," and he did not name him.