Musnad Ahmad 5:346

Abdullah narrated to us, Abdullah ibn ‘Awn narrated to me, Mubarak ibn Sa‘id—the brother of Sufyan—narrated to us, from his father, from Habib ibn Abi Thabit, from ‘Abd Khayr al-Hamdani, who said: I heard ‘Ali saying upon the pulpit: "Shall I not inform you of the best of this nation after its Prophet?" He said: Then he mentioned Abu Bakr. Then he said: "Shall I not inform you of the second?" He said: Then he mentioned ‘Umar. Then he said: "If I wished, I would have informed you of the third." He said: Then he remained silent, and we perceived that he meant himself. I said: "Did you hear him say this?" He said: "Yes, and by the Lord of the Ka‘bah, or else may both my ears be deaf."