Sayyiduna Abu Hurairah (may Allah be pleased with him) reports that the Noble Prophet (peace and blessings be upon him) said: “Does any one of you who raises his head before the Imam (in bowing or prostration) not fear that Allah may turn his head into the head of a donkey, or his appearance into the appearance of a donkey?”
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1353