What does one do if he forgets the final prostration in the prayer?

Question:
One of the brothers says: An imam prayed with the people and left out the final prostration, then he said the closing salam. Then they alerted him. He then said: He said the takbir and prostrated three times and said the salam. Is this action valid or invalid?

Answer:
No, he prostrates the prostration he left out, then he says the salam, then he prostrates two prostrations for forgetfulness.
Is it not obligatory for him to perform a second rak'ah?
The Shaykh: It is not obligatory; the correct view is that it is not obligatory. He prostrates the prostration he left out, then he says the salam, then he prostrates the two prostrations. This is the most preferred. If he prostrates for forgetfulness, then the prostration he left out, then prostrates a prostration for forgetfulness, then says the salam; this is valid; because delaying it is only the most preferred option.
He now prostrated three prostrations together... and said the salam?
The Shaykh: There is no harm. He must recite the tashahhud after the prostration he missed, then he performs it. He recites the tashahhud, then he prostrates for forgetfulness before the salam or after it, and after it is better.
What did he do...?
The Shaykh: He repeats the prayer if the interval was long. If the interval was not long, he recites the tashahhud, then he says the salam, then he prostrates two prostrations for forgetfulness.