The ruling on the imam if he forgets a prostration and then makes it up after the prayer

Question:
We received a message from the sender, the student Muhammad Nur Musa 'Urfah, from Kassala, in Sudan. He says: If the imam leaves out one prostration, and the prayer was 'Isha', and he was in the last rak'ah, then he said the closing salam, and some of the ma'mum said to him: "O Shaykh, you left out the prostration, so remember it," then he remembered, and he performed the prostration while sitting in his place of prayer, then he prostrated for forgetfulness after the salam. Is this manner valid or not? And if it is not valid, what does he do, with the evidence?

Answer:
This issue is subject to scholarly disagreement. The well-known view among some scholars is that he performs a rak'ah; the entire rak'ah is invalidated, and he performs a complete rak'ah, if he remembered it and was reminded of it after the salam. Some scholars held that he performs a complete rak'ah, then says the salam, then prostrates for forgetfulness after that; because he said the salam while the prayer was deficient. Some of them held that it suffices for him to perform the prostration, then say the salam, then prostrate for forgetfulness. And if he prostrates before the salam, there is no harm.
And given that it is important, and that it requires investigation and care, I see that it should be postponed to another night, to another session, in which we will clarify the correct view with the evidence, if Allah wills. Yes.