Musnad Ahmad 8:161

Abu Ahmad al-Zubayri narrated to us, Abdullah—meaning Ibn Habib ibn Abi Thabit—narrated to us, from Hamzah ibn Abdullah, from his father, from Sa’d, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) went out on the expedition of Tabuk, he left Ali behind. So he (Ali) said to him, "Are you leaving me behind?" He said to him, "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?"