Musnad Ahmad 8:144

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from Mus'ab ibn Sa'd, from Sa'd ibn Abi Waqqas, who said: The Messenger of Allah, peace and blessings of Allah be upon him, left Ali ibn Abi Talib behind during the expedition of Tabuk. He (Ali) said: "O Messenger of Allah, do you leave me behind with the women and the children?" 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?"