Abu Hurairah (May Allah be pleased with him) said:
The Prophet (ﷺ) said, "Does he who raises up his head before the Imam not fear that Allah will make his head that of a donkey or make his appearance similar to that of donkey?"[Al-Bukhari and Muslim].
App reference: Book 18, Hadith 241
Arabic/English book reference: Book 18, Hadith 1751