Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 337
Prohibition of raising one's Head before the Imam

1751. Abu Hurairah (May Allah be pleased with him) said: The Prophet (PBUH) 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].

Commentary:  This Hadith has a stern warning for those who take precedence over the Imam in the course of Salat. The faces of such will be turned by Allah into those of donkeys, and that is least difficult for Him. Sheikh Ibn Hajar Al-Haithami mentioned in his Al-Mu`jam that some people's faces have really been turned into those of donkeys. It is, therefore, essential for everyone who offers Salat in congregation, to follow the actions of Imam in Salat. To do anything before the Imam in the course of Salat, is a great sin and some scholars hold that such Salat is valid but the one who does so will be deemed sinful. Imam Ibn Hanbal, however, regards this Salat as invalid.