What is the ruling on the prayer of a man who intended to perform the missed Maghrib prayer with the 'Isha prayer, but missed one rak'ah, and completed his prayer such that the form of the Maghrib prayer changed, becoming one rak'ah then two rak'ahs?

The prayer of this person is valid because he followed an imam who was performing a different prayer, and this is permissible. His sitting for the tashahhud in the first rak'ah was merely following the imam. This is what the Permanent Committee for Scholarly Research and Ifta' ruled, stating: "If a latecomer prays Maghrib, performing three rak'ahs behind an imam who is praying Isha, his prayer is valid."