What should the worshiper do if he precedes his imam in bowing (ruku') and prostration (sujud) and then remembers that?

The worshipper must follow his Imam and perform an extra rak'ah after his Imam says the taslim, because the rak'ah in which he was preceded is invalid. He must also perform the prostrations of forgetfulness for the invalid extra rak'ah. He may prostrate before or after the taslim. Some scholars prefer the prostrations of forgetfulness to be after the taslim because they resulted from an addition.