Musnad Ahmad 30:2368

Isma'il narrated to us, from Yunus 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 in his prayer before the Imam not fear that Allah might transform his form into the form of a donkey?"