Musnad Ahmad 30:2756

Muhammad ibn Ja‘far and Hajjaj narrated to us, they said: Shu‘bah narrated to us, from Muhammad ibn Ziyad, he said: I heard Abu Hurayrah saying: The Messenger of Allah (peace and blessings of Allah be upon him) said—Hajjaj said: Or he said: Abu al-Qasim said—: "Does one of you not fear—or is one of you not afraid—that Allah might make his head the head of a donkey—or his image the image of a donkey—if he raises his head before the Imam, while the Imam is prostrating?"