Muhammad bin Ziyad said: I heard Sayyiduna Abu Hurayrah (may Allah be pleased with him) say: I heard Abu al-Qasim (peace and blessings be upon him) say: "Does the one who raises his head before the imam not fear that Allah Almighty will turn his head into that of a donkey?"
Shu'bah said in his narration: "Or (turn) his form into the form of a donkey."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2600