What is the ruling on the prayer of the congregants if the Imam prostrates once in the last rak'ah and then says the tasleem, and then one of the worshipers alerts him after most of them have departed?

If the Imam remembers the missed prostration soon after, he should perform what is incumbent upon him, and his responsibility and the responsibility of the congregants are absolved. However, if he remembers it after a long separation, then he and the congregants must repeat the prayer. The criterion for a long separation is custom and common practice. As for the congregants who left the mosque before the completion of the prayer, they must repeat it once they become aware of the oversight.