The Rule for Following the Imam in Prayer and Adding to It

Question:
The questioner asks: What is the criterion for following the imam? Because some people say—and I have heard them say this—that we follow the imam even if he stands for a fifth rak'ah, and we know it is the fifth, taking this from the hadith: “The imam was appointed to be followed.”

Answer:
Following the imam in prayer regarding an addition: following the imam is obligatory in prayer. This means saying takbir after the imam’s takbir, bowing after his bowing, rising after his rising, and prostrating after his prostration. Thus, the imam’s actions and words become the ma’mum’s actions and words after the imam. As for this addition, it is not part of the prayer, so one does not follow him in anything that is not part of the prayer; rather, it is an addition. It is not permissible for him to follow him in the addition if he knows it. If the ma’mum follows the imam in the addition while knowing it, the following is not permissible and his prayer is invalid. Yes.