What must the Imam and the congregants do if the Imam forgot the second prostration in one of the rak'ahs of the Fajr prayer and remembered this after finishing the prayer and the people had dispersed?

Prostration (sujud) is one of the pillars of prayer. Its omission due to forgetfulness cannot be compensated for, and its intentional omission invalidates the prayer. The worshiper praying behind an imam (ma'mum) must alert their imam if the imam omits a prostration. If the imam does not realize, the ma'mum should prostrate, and their prayer will be valid. However, if the ma'mum follows the imam [in omitting the prostration], their prayer becomes invalid. As for an imam who does not remember having omitted a prostration until after a long interval (such as people dispersing), their prayer is invalid, and they must repeat it.