The ruling on the prayer of one who forgets to recite al-Fatihah

Question : To proceed: Honorable Shaykh, may Allah reward you and benefit us with your knowledge. The questioner says: What is the ruling on the prayer of a person who forgets to recite al-Fatihah in one of the rak'ahs of the obligatory prayer?

Answer : If a person leaves off the recitation of al-Fatihah in one of the rak'ahs, and he remembers before he reaches the position of the second rak'ah, it is obligatory for him to go back and recite al-Fatihah, then complete the prayer and prostrate for forgetfulness after the taslim. If he does not remember until he reaches the position of the second rak'ah, the second rak'ah becomes the first, so he completes the prayer based on it and prostrates after the taslim. This is because the recitation of al-Fatihah is a pillar without which the prayer is not valid. As for if he leaves off any of the other dhikr, such as if he forgets to say "Subhan Rabbi al-Azim" in the ruku' and does not remember until he has risen from the ruku', he does not go back to the ruku'; he continues in his prayer and prostrates for forgetfulness before the taslim.
Did you understand, my brothers? Why did we say in the first case that he prostrates after the taslim, and in the second case that he prostrates before? Because his prostration in the first case was for an addition, and his prostration in the second case was for a deficiency. The rule is that the prostration for forgetfulness, if it is for an addition, is after the taslim, and if it is for a deficiency, it is before the taslim.