Prostrating Before the Imam in Congregational Prayer - Jurisprudential Answers - Miscellaneous Issues, Vol 3

Ahmed al-Hasan Response:

I explained this matter in detail in Al-Shara'i. The text is: [Following the imam is obligatory. If the follower raises his head while unaware of the imam's state, he returns in order to follow him; if he did so forgetfully, he also returns to follow him. Likewise, if he goes into bowing or prostration before the imam thinking that the imam has already gone down, he returns and follows the imam. But if he deliberately bows before the imam while knowing the imam's state, then if he intended to separate from the congregation he completes his prayer individually; otherwise his prayer is invalid.] In your case, if it happened inadvertently, you should have followed the imam: if you prostrated or bowed before him, you should have raised your head immediately and then followed him, bowing or prostrating with him. If, however, you delayed following because you did not know the legal ruling-as appears from your description-there is nothing upon you so long as you ultimately followed the imam. Your prayer is valid.