The ruling for the one who joins the prayer late if the imam adds a rak'ah out of forgetfulness

Question:
If a Muslim arrives while the imam is praying the midday prayer and catches up with him for the second rak'ah, but the imam makes a mistake in the prayer and prays five rak'ahs, does his prayer remain valid if he does not perform the rak'ah he did not catch up with the imam for? And if the imam prostrates for forgetfulness, should he prostrate with him? Please clarify this issue, may Allah reward you with good.

Answer:
There is scholarly disagreement among the scholars (may Allah have mercy on them) regarding this issue. Some said that the one who joins the prayer late may suffice with the extra rak'ah, while others said that he may not suffice with it. The correct view is that he may not suffice with it, because making up the missed prayer is done after the closing salam. Therefore, when his imam says the salam, he must stand up and perform what he missed. He is not to follow the imam in the addition; rather, he should sit until the imam says the salam. When the imam says the salam, he stands up and makes up what is required of him. Likewise, the other followers of the imam are not to follow the imam in the addition; rather, they should alert him. If he responds to them, fine; otherwise, they wait for him and do not follow him if they know that it is an addition. However, if someone follows him out of ignorance of the legal ruling or out of ignorance that it is an addition, his prayer is valid. And the one who joins the prayer late, if the imam prostrates for forgetfulness, should prostrate with him. Then, when the imam says the salam, he stands up and performs what is required of him. And success is from Allah.