What is the ruling on preceding the Imam? Is the Salah (prayer) of a person who precedes him valid?
Preceding the Imam is prohibited. It is one of the major sins because of the punishment set for those who do that. It was authentically reported that the Prophet (peace be upon him) said:
"Isn't he who raises his head before the Imam afraid that Allah may transform his head into that of a donkey or his figure (face) into that of a donkey?"
This Hadith was reported by Al-Bukhari.
There is a difference concerning the validity of the Salah of a person who does this. The soundest opinion is that if one intentionally precedes the Imam, his Salah is invalid, but if he precedes him by mistake, then his Salah is valid and he has to go back and follow his Imam. (Part No. 7; Page No. 327) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.