The Ruling on the One Praying an Obligatory Prayer Behind the One Praying a Voluntary Prayer, and the Reverse

Question:
The questioner says: Honorable Shaykh, I am confused about the ruling on praying the obligatory prayer behind someone who is praying a voluntary prayer. Is it permissible to pray in such a case, and conversely, is it permissible for the one praying a voluntary prayer to pray behind the one praying the obligatory prayer?

Answer:
Yes, this is mentioned in the hadiths that 'Umar ibn al-Khattab (may Allah be pleased with him) used to pray with the Prophet (peace and blessings be upon him) and then go out and pray with his people, and the Prophet (peace and blessings be upon him) approved of that for him. This is regarding the obligatory prayer behind the voluntary prayer, and likewise the voluntary prayer behind the obligatory prayer. It is permissible for one to pray intending a voluntary prayer behind someone who is praying the obligatory prayer. This is because a man entered the mosque after the Messenger (peace and blessings be upon him) had finished the prayer, so he missed the prayer. The Prophet said: The one who entered has the obligatory prayer, and the one who prayed with him has a voluntary prayer. This is regarding the voluntary prayer behind the obligatory prayer. Likewise, the Prophet (peace and blessings be upon him) in Mina used to pray with his companions in the Musjid al-Khaif.
When he finished, two men were sitting behind the praying people, so he called for them and said to them: Why did you not pray with us? They said: O Messenger of Allah, we prayed in our tents. He said: Do not do that. You prayed in your tents, then you came and the prayer was being established, so pray with them, for it will be a voluntary prayer for you. This is evidence that it is valid for the one praying a voluntary prayer to pray behind the one praying the obligatory prayer. Yes.