Musnad Abu Ya'la al-Mawsili 6:21

Ubayd Allah ibn Mu'adh narrated to us, my father narrated to us, Shu'bah narrated to us, from Ali ibn Zayd, who said—Shu'bah said before he became confused—I heard Sa'id ibn al-Musayyib say: I heard Sa'd ibn Malik say: The Prophet, peace and blessings of Allah be upon him, left Ali behind. He (Ali) said: "Are you leaving me behind?" 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?" He said: "I am pleased, I am pleased."