If the ma'mum forgets to recite al-Fatihah, what is the ruling on his prayer?

Question:
If the ma'mum forgets to recite al-Fatihah, what is his ruling?

Answer:
Al-Nawawi (may Allah have mercy on him) mentioned that there are two opinions among the scholars regarding the original issue:
The first is that the ruling for one who forgets al-Fatihah is the same as the ruling for one who forgets any of the other pillars. If he remembers it during the bowing or after it, before standing up for the second rak'ah, he must return and recite al-Fatihah and what follows it. If he remembers it after standing up for the second rak'ah, the rak'ah in which he did not recite al-Fatihah is void, and the second rak'ah takes its place. If he does not remember it until after the taslim, and a short interval has passed, he returns to the prayer and performs a rak'ah in place of the rak'ah from which he omitted al-Fatihah, and he performs the prostration for forgetfulness if he was a masbuq. If he was not a masbuq, he does not perform the prostration for forgetfulness; rather, the imam bears it for him. If a long interval passes before he remembers, he must restart the prayer. Al-Nawawi stated that this opinion is the new view of al-Shafi'i, and that it is the most correct according to the consensus of the companions, except that he was certain that one performs the prostration for forgetfulness without distinguishing between cases, whereas the correct view is to distinguish as previously mentioned.
The second opinion is that if one leaves off the recitation of al-Fatihah out of forgetfulness until he bows or says the taslim, the recitation is dropped from him and his prayer is complete. Al-Nawawi attributed this to a group of the Shafi'is.
This opinion is stronger in my view specifically for the ma'mum, and the first opinion is stronger for the imam and the one praying alone.
The reason for this is that the ma'mum is commanded to follow his imam and imitate him in the bowing and other actions of the prayer. So if his imam bows, he must follow him in the bowing, even if he has forgotten to recite al-Fatihah. It is not permissible for him to stand up to recite it while his imam is bowing, due to the generality of the saying of the Prophet (peace and blessings be upon him): . The "fa" (so) indicates sequence with continuity, and the apparent meaning of the hadith includes one who left off al-Fatihah out of forgetfulness and others. We only made the recitation of al-Fatihah obligatory for the ma'mum due to the generality of the texts indicating its obligation. So if he forgets it until his imam bows, it is dropped from him due to the excuse of forgetfulness and the obligation to follow.
The one who is ignorant of the obligation of reciting al-Fatihah for the ma'mum has the same ruling as the one who forgets, as it appears to me, whether he is a follower of one who did not see it as obligatory, because the evidence for it was not clear to him, or an independent jurist who did not see it as obligatory. This is because both are excused: either by the excuse of ignorance or by the excuse of forgetfulness. Their situation resembles the situation of one who catches up with the imam while he is bowing. It was previously mentioned that the rak'ah suffices for him and the recitation is dropped from him. In fact, the ignorant person and the one who forgets are more deserving here of the rak'ah sufficing for them, because the one who catches up with the imam while he is bowing has missed both the recitation and standing for it, whereas the ignorant person and the one who forgets here have only missed the recitation. And Allah knows best.
As for the prostration for forgetfulness in the case of the one who forgets here, it follows the distinction mentioned in the first opinion. As for the ignorant person, he does not perform the prostration for forgetfulness in any case.