What should the person praying behind an imam do if he recites a verse in which there is a sajdah (prostration)?

Praise be to Allah.

You should not prostrate, because following the imam is obligatory and the prostration of recitation is Sunnah. If a person is praying behind an imam, it is not permissible for him to prostrate, and if he prostrates deliberately although he knows that that is not permissible, then his prayer becomes invalid. End quote.