The ruling on the prayer of one who is certain that the imam added a rak'ah

The questioner:
Your Excellency the Shaykh, may Allah bless you and your uncle, we ask your permission to ask a few questions. This questioner says: We prayed the 'Asr prayer in a mosque, and the imam stood up and performed a fifth rak'ah. The worshippers alerted him, and they were certain that he had added a fifth, but he did not go back. Some of them sat down, and some stood up with him. After the prayer, a dispute arose among the worshippers regarding the validity of the prayer, so what is your guidance, may Allah reward you?

The Shaykh:
As for those who were certain that it was an extra rak'ah, it is not permissible for them to stand up. If they stood up intentionally, their prayer would be invalid. But as for those who stood up with him out of ignorance, not knowing the ruling, they are excused for their ignorance and their prayer is valid. As for the one who stood up with him while knowing that it was extra and knowing the legal ruling, his prayer is invalid. He has to repeat it.

The questioner:
And the imam, the Shaykh.

The Shaykh:
The imam does not assert the correctness of his own action, so his prayer is valid. He did not persist, but he asserted the correctness of his own action while being mistaken. However, he did not do it intentionally, so as long as he did not do it intentionally, his prayer is valid. Yes.