The ruling on one who forgets a pillar of the prayer and then remembers it

Question:
Honorable Shaykh, if the imam forgets a pillar of the prayer, then remembers it in the following rak'ah, or after the prayer, what does he do?

Answer:
If he forgets a pillar, such as bowing, prostration, or reciting Surat al-Fatihah, he is required to go back and perform it, unless he has entered the second rak'ah by standing for it. In that case, the previous rak'ah is voided, and the second takes its place. He completes his prayer in this manner, and then he performs the prostration for forgetfulness. Yes.
As for if he has not entered the second rak'ah, he goes back and performs the pillar he left and the pillars after it. Then, when he intends to conclude the prayer, he performs the prostration for forgetfulness, either before the taslim or after the taslim; all of this is permissible, if Allah wills. Yes. And Allah the Exalted knows best, and may Allah send prayers and peace upon our Prophet Muhammad, and upon his family and his Companions.