Musnad Ahmad 30:2939

'Abd al-Rahman narrated to us, Hammad narrated to us, from Muhammad, who said: I heard Abu Hurayrah saying: I heard Abu al-Qasim (may Allah bless him and grant him peace) saying: "Does one of you not fear, when he raises his head while the Imam is prostrating, that Allah might make his head the head of a donkey?"