The ruling on one who forgets the prostration for forgetfulness

Question:
The questioner says: Honorable Shaykh, an imam in a four-rak'ah prayer sat in the third rak'ah for the final tashahhud, but the ma'mum stood up for the fourth rak'ah, and the imam stood up with them and completed the last rak'ah, but he did not perform the prostration for forgetfulness, so he said the taslim. Is this prayer valid, or is it obligatory for him to perform the prostration for forgetfulness?

Answer:
It is obligatory for him to perform the prostration for forgetfulness, but if a long time has passed, if he left it and a long time has elapsed, the prayer is valid. Otherwise, it is obligatory for him to prostrate for forgetfulness after the taslim, or when the time is close to the taslim. But if a long time has passed and the interval is long, his prayer is valid, and the prostration for forgetfulness is dropped from him due to the length of the interval. Yes.