Musnad Abu Ya'la al-Mawsili 3:84

Ubayd Allah narrated to us, Ghundar narrated to us, Shu'bah narrated to us, from al-Hakam, from Mus'ab bin Sa'd, from Sa'd bin Abi Waqqas, who said: The Messenger of Allah (may Allah bless him and grant him peace) left Ali bin 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 (the Prophet) 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?"