Musnad Abu Ya'la al-Mawsili 6:10

Abu Khaythama narrated to us, Affan narrated to us, Hammad narrated to us, from Ali bin Zayd, from Sa’id bin al-Musayyib, who said: I said to Sa’d bin 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: I said, "The statement of the Messenger of Allah (peace and blessings of Allah be upon him) to Ali when he left him 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 the 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: Then Ali turned away quickly, and it is as if I am looking at the dust of his feet rising. Hammad said: Ali returned quickly.