Musnad Abu Ya'la al-Mawsili 155:311

Amr al-Naqid narrated to us, Qaran bin Tammam narrated to us, from Muhammad bin Abi Humayd, from Musa bin Wardan, from Abu Hurayrah, who said: We were with the Prophet, may Allah bless him and grant him peace, when a man stood up and said: "O Messenger of Allah, how incapable—or he said: how weak—is so-and-so." The Prophet, may Allah bless him and grant him peace, said: "You have backbitten your companion and eaten his flesh."