Abu Hurairah reported the Messenger of Allah ﷺ as saying; Does he who raises his head while the Imam is prostrating not fear that Allah may change his head into a donkey’s or his face into a donkey’s face. (Using translation from Abū Dāʾūd 623)
"أما يخشَى أحدُكم إذا رفعَ رأسَهُ قبلَ الإِمام أن يجعلَ اللهُ رأسَهُ رأسَ حمارٍ، أوْ يجعلَ اللهُ صُورته صورةَ حمارٍ".