The Ruling on Praying the Obligatory Prayer Behind One Who Is Praying a Voluntary Prayer

Question:
He says: Honorable Shaykh, may Allah grant you success. A man entered the mosque and the prayer had ended. He found a man who had entered the mosque and the congregational prayer had ended, and he found a man praying the sunnah (recommended practice) after the prayer. So he joined him to pray the obligatory prayer. Is his action valid? And does the one praying the voluntary prayer indicate to him that he is praying a voluntary prayer?

Answer:
This action is valid; there is no obstacle. The one who must pray the obligatory prayer may join the one praying the voluntary prayer in congregation; there is no obstacle. The Companions (may Allah be pleased with them) used to pray behind Mu'adh (may Allah be pleased with him), and he had prayed the obligatory prayer behind the Messenger (peace and blessings be upon him) and then prayed a voluntary prayer with them, and the Messenger (peace and blessings be upon him) approved him for that. Yes.