What should one do who forgets to perform a prayer in congregation, believing that he has already prayed it, and what is the sin incurred by that?

Prayer is a great act of worship; it is the second pillar of Islam, the pillar of the religion, and the first thing a servant will be questioned about on the Day of Judgment. Therefore, it is essential to give it importance and perform it in the most complete manner. Prayer must be performed on time and in congregation, as congregational prayer is obligatory unless there is an valid excuse. If a prayer is forgotten, there is no sin, but it must be performed immediately upon remembrance.