The World Federation One Stop Fiqh

Ruling 1451

If a follower inadvertently raises his head from rukūʿ before the imam, then based on obligatory precaution, in the event that the imam is in rukūʿ, he must go back into rukūʿ and raise his head with the imam; in this case, performing the additional rukūʿ, which is a rukn, does not invalidate the prayer. If he intentionally does not go back into rukūʿ, then based on obligatory precaution his congregational prayer becomes invalid although his prayer [performed on his own] is valid as per the details mentioned in Ruling 1403. However, if he goes back into rukūʿ but before he joins the imam in rukūʿ the imam raises his head, then based on obligatory precaution his prayer is invalid.