What is incumbent upon the Imam and the congregants if the Imam forgot to recite a verse from Al-Fatihah and was not prompted until he completed the prayer, or if he forgot and was prompted?

Scholars have differed regarding the ruling on correcting the Imam if he makes a grammatical error (laḥn) in Al-Fatiḥah that alters its meaning, or if he forgets a verse from it. The Malikis and Hanbalis obligated correction, while the Shafi'is and Hanbalis in one narration did not obligate it. The preponderant opinion is that it is obligatory to correct the Imam in this situation.

Therefore, if the congregants correct the Imam when he omits a verse from Al-Fatiḥah, and he then returns to it and recites it, they have done what was obligatory upon them. If they do not correct him, Shafi'i jurists explicitly stated that it is necessary for them to separate from him.

As for the Imam, he must perform an additional rak'ah to replace the one in which he forgot a verse from Al-Fatiḥah, provided he remembers soon and performs the prostration of forgetfulness (sujūd al-sahw). If the congregants do not separate from their Imam and continue following him, they must repeat the prayer. The same applies to the Imam if he does not perform an additional rak'ah to replace the one in which he forgot a verse from Al-Fatiḥah and a long time has passed.