Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Isn't he who raises his head before the Imām afraid that Allah may transform his head into that of a donkey or his shape into that of a donkey?"
شرح الحديث :
The purpose of having the Imām in prayer is to follow him in his movements, in such a way that the movements of those praying behind him come after his movements, and in this way, they fulfil the true sense of following the Imām. However, if the one who is being led in prayer precedes the Imām, then the purposes of leading people in prayer are not realized. For this reason, this Hadīth informs of a serious threat to the one who raises his head before his Imām that Allah will transform his head into that of a donkey, or change his shape into that of a donkey. His head will change from the most beautiful form to the ugliest form, as a punishment to this body organ that was raised and disturbed the prayer.