Musnad Ahmad 30:417

'Abd al-A'la narrated to us, from Yunus—meaning Ibn 'Ubayd—from Muhammad ibn Ziyad, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Does the one who raises his head before the Imam, while he is with the Imam, not fear that Allah might transform his form into the form of a donkey?"