Musnad Ahmad 31:282

Waki' narrated to us, Fudayl ibn Marzuq narrated to us, from 'Atiyyah al-'Awfi, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to 'Ali: "You are to me in the position of Aaron to Moses, except that there is no prophet after me."