Musnad Abu Ya'la al-Mawsili 160:166

Abu Bakr narrated to us, Zayd ibn al-Hubab narrated to us, Muhammad ibn Salih al-Tammar al-Madani narrated to us, Muslim ibn Abi Maryam narrated to us, from Sa'id ibn Abi Sa'id al-Maqburi, who said: We were with Abu Hurayrah when al-Hasan ibn Ali came and greeted us. We returned the greeting to him, but Abu Hurayrah did not know. He (al-Hasan) passed by, so we said, "O Abu Hurayrah, this is al-Hasan ibn Ali; he greeted us." He said, so he followed him and caught up with him. He said, "Peace be upon you, O my master." He said, "I heard the Messenger of Allah, may Allah bless him and grant him peace, say: 'Indeed, he is a master.'"