The imam completed the fifth rak'ah in the 'Asr prayer, so what is the ruling for the imam and the ma'mum?

Question:
May Allah reward you with good and do you good. After this, a message was sent by a listener who indicated his name with the letters M.M.M.M.A. He says: We were praying the 'Asr prayer in a mosque near our house, and while we were in the prayer, in the fourth rak'ah, the imam stood up for the fifth rak'ah by mistake. The ma'mum were confused; some said, and some stood up with the imam. Some sat down, so we alerted the imam by saying "Subhan Allah." And it is strange that the imam did not sit down but rather completed the fifth rak'ah. Some of the ma'mum followed him and prayed with him, while others did not pray the fifth rak'ah with the imam and remained sitting until the imam reached the tashahhud, prostrated the sujud al-sahw, and said the taslim. The worshippers differed on the validity of the prayer. A man, who is of the type found in some of our mosques, stood up and said that those who completed the prayer with the imam and prayed the fifth rak'ah have an invalid prayer, while those who did not complete it with the imam and remained sitting have a valid prayer. What is your honorable opinion on this? May Allah reward you with good.

Answer:
If the imam stands up for the fifth rak'ah and the ma'mum alert him, and he is not certain of his own correctness, it is obligatory for him to return. However, if he is certain of his own correctness, he continues. But as for the ma'mum, if one knows that this is the fifth rak'ah and knows that following the imam is not permissible, yet he follows him while knowing that it is the fifth and that following him is not permissible, then his prayer is invalid. As for the one who followed him while ignorant that it was the fifth, or ignorant of the ruling on following the imam in this case, his prayer is valid. So this distinction is necessary. As for those who sat down while knowing that this was the fifth, and they sat down, they have done correctly. They have the choice: if they wish, they say the taslim for themselves, and if they wish, they wait until he says the taslim so they say it with him. But the imam must prostrate for forgetfulness. If the imam did not know that it was an extra rak'ah, but rather thought the prayer was not completed, and did not intend the addition, he must prostrate for forgetfulness, and this addition has no ruling because he did not intend it. He must prostrate for forgetfulness after the taslim, and the ma'mum must prostrate with him following him. Yes.