Musnad Ahmad 8:52

'Affan narrated to us, Hammad—meaning Ibn Salamah—narrated to us, 'Ali ibn Zayd informed us, from Sa'id ibn al-Musayyib, who said: I said to Sa'd ibn Malik, "I want to ask you about a hadith, but I am hesitant to ask you about it." He said, "Do not do that, O son of my brother. If you know that I have knowledge, then ask me about it and do not be hesitant with me." He said: So I said, "The statement of the Messenger of Allah (may Allah bless him and grant him peace) to 'Ali when he left him behind in Medina during the expedition of Tabuk." Sa'd said: The Prophet (may Allah bless him and grant him peace) left 'Ali behind in Medina during the expedition of Tabuk. He said, "O Messenger of Allah, are you leaving me behind with those who stay behind, among the women and children?" He said, "Are you not pleased that you should be to me in the position of Aaron to Moses?" He said, "Yes, O Messenger of Allah." He said: So 'Ali turned back quickly, as if I am looking at the dust of his feet rising. Hammad said: And 'Ali returned quickly.