Musnad Ahmad 30:3415

Yazid narrated to us, Shu'bah narrated to us, from Muhammad ibn Ziyad, from Abu Hurayrah, from the Prophet, may Allah bless him and grant him peace, who said: "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, or his image the image of a donkey?"