Musnad Ahmad 8:108

Sufyan ibn 'Uyaynah narrated to us, from 'Ali ibn Zayd, from Sa'id ibn al-Musayyib, from Sa'd: That the Prophet, peace and blessings of Allah be upon him, said to 'Ali: "You are to me in the position of Aaron to Moses." It was said to Sufyan: "Except that there is no prophet after me?" He said: "Yes."