If the Imam prolongs his bowing (ruku') or prostration (sujud), what should the worshipper (ma'mum) do?

If the worshiper (ma'mum) is certain that an extra bowing (ruk'u) or prostration (sujud) has occurred, he must alert the Imam by saying "Subhanallah" (Glory be to Allah). It is not permissible for him to follow the Imam in that extra movement. If he does follow him, while knowing that it is forbidden to do so in this situation, his prayer becomes void.