It is narrated from Sayyiduna Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Does any one of you not fear that, for raising his head from prostration before the imam, Allah Almighty will change his head into the head of a donkey?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2601