Imam Al-Nawawi’s Riyad-us-Saliheen
Prohibition of raising one’s Head before the Imam
1751. Abu Hurairah (May Allah be pleased with him) said: The Prophet (sallallaahu alayhi wa sallam) 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].