The ruling on the prayer of one who prays an obligatory prayer behind one who is praying a voluntary prayer

Q: What is the ruling on the prayer of one who prays the obligatory prayer behind one who is praying a voluntary prayer?

A: The ruling in this matter is that the prayer is valid; because it is established from the Prophet (peace and blessings be upon him) that in some of his journeys, he prayed the prayer of fear with a group of his Companions for two rak'ahs, then he prayed with the other group for two rak'ahs, so the second prayer for him was a voluntary prayer. Likewise, it is established in the two Sahihs from Mu'adh (may Allah be pleased with him) : that he used to pray with the Prophet (peace and blessings be upon him) the obligatory part of the 'Isha' prayer, then he would go and pray the obligatory prayer with his congregation, so it was an obligatory prayer for them and a voluntary prayer for him.