Musnad Ahmad 5:296

Aswad ibn Amir narrated to us, Abd al-Hamid ibn Abi Ja’far—meaning al-Farra’—narrated to me, from Isra’il, from Abu Ishaq, from Zayd ibn Yuthay’, from Ali, who said: It was said, "O Messenger of Allah, whom shall we appoint as leader after you?" He said, "If you appoint Abu Bakr, you will find him trustworthy, ascetic in the world, and desirous of the Hereafter. If you appoint Umar, you will find him strong and trustworthy, fearing no blame from any critic for the sake of Allah. And if you appoint Ali—though I do not think you will do so—you will find him a guide who is guided, who will lead you upon the straight path."