What is incumbent upon someone who prayed the obligatory prayer behind a person performing a supererogatory prayer, whom he joined when that person stood for the second rak'ah of the supererogatory Maghrib prayer, then the person performing the supererogatory prayer completed the full rak'ahs of Maghrib when he sensed the worshiper joining him?

According to some scholars, and this is the most preponderant view, it is permissible for someone praying an obligatory prayer to pray behind someone performing a supererogatory prayer, citing the hadith of Mu'adh as evidence. Therefore, it is valid for a person to pray the obligatory Maghrib prayer behind someone who is performing its ratibah (supererogatory prayer), provided that the imam who is performing the supererogatory prayer does not add to what he intended for his prayer. If he adds, the prayer of the ma'mum (follower) is invalidated if he follows him while being aware of the addition, and he must repeat it. However, if the ma'mum is unaware that the imam has added, then he may continue to follow him.