Musnad Ahmad 5:371

Abdullah narrated to us, Suwayd ibn Sa'id narrated to me, al-Sabi ibn al-Ash'ath narrated to us, from Abu Ishaq, from 'Abd Khayr, from Ali: "Shall I not inform you of the best of this nation after its Prophet? Abu Bakr, and the second is Umar, and if I wished, I would name the third." Abu Ishaq said: 'Abd Khayr spelled it out so that they would not doubt what he said: "Ali."