Musnad Ahmad 8:94

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Qatadah and 'Ali ibn Zayd ibn Jud'an, they both said: Ibn al-Musayyib narrated to us, a son of Sa'd ibn Abi Waqqas narrated to me a hadith from his father, who said: I entered upon Sa'd and said: "A hadith I was told about you, regarding when the Messenger of Allah (peace and blessings of Allah be upon him) appointed 'Ali as his successor over Medina." He said: He became angry and said: "Who told it to you?" I disliked informing him that his son had told it to me, lest he become angry with him. Then he said: "When the Messenger of Allah (peace and blessings of Allah be upon him) went out on the expedition of Tabuk, he appointed 'Ali as his successor over Medina. 'Ali said: 'O Messenger of Allah, I would not have liked for you to go out on any campaign except that I was with you.' He said: 'Are you not pleased that you should be to me in the position of Aaron to Moses, except that there is no prophet after me?'"