Musnad Ahmad 30:416

'Abd al-A'la narrated to us, from Ma'mar, from Muhammad ibn Ziyad, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said—or Abu al-Qasim (peace and blessings of Allah be upon him) said: "Does the one who raises his head while the Imam is prostrating not fear that Allah might turn his head into the head of a donkey?"