What is the Ruling on Praying Voluntary prayer behind the One Praying Obligatory Prayer?

As for the one who is praying a voluntary prayer behind someone praying an obligatory prayer, there's no problem. There is no problem with it, because it is indicated by the Sunnah. For indeed, the Messenger ﷺ turned after (performing) the Fajr Salah in Masjid Al-Khaif in Mina, one day, when he found two men who hadn't prayed (in congregation). So he ﷺ said: "What prevented you both from praying with the people (in congregation)." They said: "O Messenger of Allah, we have already prayed in our houses." They may have prayed in their houses because of their assumption that they would not be there in time for the congregation, or for other reasons. Then, he ﷺ said: "If any of you prays in his house and then comes to the Masjid (and there is a) congregation, then pray along with them. Surely it is a voluntary prayer for you." "Indeed it", referring to the first Salah or the second? (It refers to the) second Salah because the first one fulfilled the obligatory prayer, which was completed and freed (the person from it's) obligation. The second (prayer) is voluntary for you. And they were praying behind the one who is praying the obligatory prayer. If the person who is being led in Salah is the one praying the voluntary prayer and the Imam is the one praying the obligatory prayer, then there is no problem with that, as established by this (narration of) the Sunnah.