Musnad Abu Ya'la al-Mawsili 6:51

Sa'id ibn Mutarrif al-Bahili narrated to us, Yusuf ibn Ya'qub narrated to us, from Ibn al-Munkadir, from Sa'id ibn al-Musayyib, from 'Amir ibn Sa'd, from Sa'd, that he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say to 'Ali: "You are to me in the position of Aaron to Moses, except that there is no prophet with me." Sa'id said: I wanted to hear this directly from Sa'd, so I met him and mentioned to him what 'Amir had mentioned to me. I said to him: "Did you hear it?" He said: "Yes." I said: "You heard it?" He put his two fingers into his ears and said: "Yes, and if not, then may they both go deaf."