What is the ruling regarding the prayer of congregants whose imam did not perform another rak'ah after forgetting to recite Al-Fatihah in the second rak'ah of Fajr prayer? Do they need to repeat the entire prayer, or perform one rak'ah and prostrate for forgetfulness?

The Imam must revert to the opinion of those among the congregants (Ma'mumin) who correct him by saying "SubhanAllah," if they are two or more, unless the Imam is certain of his own correctness. If the Imam does not respond to the congregants' correction after he has omitted reciting Al-Fatiha and continues with the recitation, then the congregants must separate from him and complete their prayers individually. They must repeat their prayer if they do not do so. Likewise, the Imam must repeat the prayer if he omits a pillar (Rukn), unless he is certain of his own correctness when they corrected him. If a long time elapses, then everyone must repeat the prayer. However, if the Imam has recited silently (Asrara) when he should have recited aloud (Jahara), the prayer is not invalidated and separation is not obligatory, as reciting aloud and silently are among the Sunnahs of prayer.