The ruling on a person praying an obligatory prayer behind a person praying a voluntary prayer

Question:
May Allah reward you, Honorable Shaykh. A questioner says: I was praying the voluntary greeting of the mosque, when a person came and stood behind me. When I finished the greeting of the mosque, he stood up and performed a third rak'ah, as if he were praying the obligatory Maghrib prayer. So, is it permissible for a person praying an obligatory prayer to follow someone like me in this case? And is it permissible for another person to follow someone who has started a prayer, so that he completes it with him?

Answer:
Yes, all of this is permissible, and praise be to Allah. It is better for him than praying alone. So if you come upon a person praying a voluntary prayer, there is no harm in you praying an obligatory prayer behind him or with him. The Companions used to pray behind Mu'adh (may Allah be pleased with him). Mu'adh used to pray with the Prophet (peace and blessings be upon him), then he would go out and pray with his people; it was an obligatory prayer for them and a voluntary prayer for him. The Prophet (peace and blessings be upon him) approved him for that, which indicates that it is permissible for a person praying an obligatory prayer to follow a person praying a voluntary prayer. And this is better than him praying alone. Yes.