Praise be to Allah.
It was narrated that Abu Hurayrah (may Allah be pleased with him) said: the Messenger of Allah (peace and blessings of Allah be upon him) said: “The Imam is appointed to be followed . When he says ‘Allahu akbar’, then say ‘Allahu akbar’, do not say it until he has said it. When he bows in Ruku`, then bow in Ruku`, and do not do it until he has done it. When he says ‘Sami`a Allahu liman hamidah,’ then say, ‘Allahumma Rabbana laka al-hamd.’ When he prostrates, then prostrate, and do not do it until he has done it. If he prays standing then pray standing, and if he prays sitting, then pray sitting, all of you.” (This version was narrated by Abu Dawud; similar reports are also narrated by Al-Bukhari and Muslim).
What we learn from this Hadith is that it is obligatory to follow the Imam , and that he is the leader in the movements, actions and words of the prayer. It is not permissible to differ from him. The best is for the follower’s movements to come immediately after the Imam’s movements , and not to delay them … This immediate following is indicated (in the original Arabic of the Hadith) by the particle Fa (immediately after), which implies fats sequence.
We also learn from this Hadith that anticipating or preceding the Imam is forbidden , and if it is done deliberately, it invalidates the prayer. Delaying following him, like preceding him, is not allowed.
The phrase “the Imam is appointed to be followed” means that the one who is following should not precede the one whom he is following or move at the same time as him; his movements should come straight after the Imam’s movements.
And Allah knows best.