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

Question:
May Allah reward you. Honorable Shaykh, this questioner says: A man prayed the voluntary prayer, and another man joined him to pray the obligatory prayer with him. Is this action permissible, provided that each one maintains his own intention?

Answer:
It is permissible for the one praying the obligatory prayer to pray behind the one praying the voluntary prayer, and vice versa. It is also permissible for the one praying the voluntary prayer to pray behind the one praying the obligatory prayer. The evidence for this is that Mu'adh (may Allah be pleased with him) used to pray with the Prophet (peace and blessings be upon him), then he would go out and pray with his people. It was a voluntary prayer for him and an obligatory prayer for them. The Prophet (peace and blessings be upon him) approved of this for him, and he only forbade him from prolonging the prayer. He forbade Mu'adh from prolonging it.