What is the ruling on the prayer of an imam who remembered in the first rak'ah of the 'Isha prayer that he had not prayed Maghrib, so he completed his prayer with the people, then when he finished and went home, he performed the Maghrib prayer?

If the Imam remembers during his current prayer that he has not prayed the obligatory prayer preceding it, he should complete his current prayer, then make up the missed prayer afterward. The obligation of observing the order between the two prayers is waived for him due to the excuse. Sheikh Ibn Uthaymeen was asked about an Imam who forgot the Asr prayer and began the Maghrib prayer, then remembered during it. He replied that the Imam should continue with the Maghrib prayer, and when he finishes it, he should pray Asr. He is not obliged to interrupt his current prayer, nor is it permissible for him to do so, because he has begun an obligatory prayer which he is required to complete unless there is a legitimate شرعي excuse.