(77) باب التَّشْدِيدِ فِيمَنْ يَرْفَعُ قَبْلَ الإِمَامِ أَوْ يَضَعُ قَبْلَهُ
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.
Reference: Sunan Abi Dawud 623
In-book reference: Book 2, Hadith 233
English translation: Book 2, Hadith 623