Sayyiduna Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "Does any one of you not fear that if he raises his head before the Imam raises his head from prostration, Allah may turn his head into the head of a donkey or his appearance into the appearance of a donkey?"
Hadith Referenceمسند اسحاق بن راهويه / كتاب الصلوٰة / 133
Hadith Takhrij«بخاري ، كتاب الاذان ، باب اثم من رفع الخ ، رقم : 691 . مسلم ، كتاب الصلاة ، باب تحريم سبق الامام الخ ، رقم : 437 . سنن نسائي : رقم : 828 . سنن ترمذي ، رقم : 582»
From another chain of narration, it is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) from the Messenger of Allah (peace and blessings be upon him) similar to the previous hadith, except that in this one he said: "Allah will change his head."
Hadith Referenceمسند اسحاق بن راهويه / كتاب الصلوٰة / 134