Musannaf Ibn Abi Shayba 36:474

Ghundar narrated to us, from Shu'bah, from al-Hakam, from Mus'ab ibn Sa'd, from Sa'd, who said: When the Messenger of Allah (may Allah bless him and grant him peace) went out on the expedition of Tabuk, he left Ali behind among the women and children. He said, "O Messenger of Allah, do you leave me behind among the women and children?" He replied, "Are you not pleased that you should be to me as Aaron was to Moses, except that there is no prophet after me?"