What does one do if he follows the imam in a fifth rak'ah while knowing it?

Question:
May Allah reward you. Honorable Shaykh, this questioner says: If one follows the imam in a fifth rak'ah, and he knows it is the fifth, what is required of him? And what is the situation of the one who joins the prayer late?

Answer:
If he follows him while knowing it is the fifth and knowing the legal ruling that it is not permissible for him to follow, his prayer is invalid. However, if he follows him and does not know it is the fifth, or he knows it but does not know that he is required to sit and not follow, he is excused due to ignorance, and his prayer is valid.