The Permissibility of a Person Praying an Obligatory Prayer Behind a Person Praying a Voluntary Prayer

We begin first with a message from Sa'd Sa'ud al-Hashal from Dhahran, in which he says: If a person arrives for the 'Isha' prayer and finds the congregation praying, so he enters the prayer with them, and then it turns out that they are in tarawih, what is the ruling on his prayer?
Answer: In the name of Allah, the Most Gracious, the Most Merciful. Praise be to Allah, and blessings and peace be upon the Messenger of Allah, and upon his family and his Companions, and upon those who follow his guidance.
As for the answer: If a person enters the prayer with the people on the nights of Ramadan, thinking they are in the obligatory prayer, and then it turns out they are in tarawih, he should continue praying with them. If the imam concludes the tarawih prayer, he should stand up and complete his prayer, and there is no harm in that. This is because, according to the correct opinion, it is permissible for one who is praying the obligatory prayer to pray behind one who is praying a voluntary prayer. Mu'adh (may Allah be pleased with him) prayed 'Isha' with his companions as a voluntary prayer while they were praying it as an obligatory one. He would pray 'Isha' with the Prophet (peace and blessings be upon him) as his obligatory prayer, then come and pray 'Isha' with his companions as a voluntary prayer while they were praying it as an obligatory one. The Prophet (peace and blessings be upon him) prayed the prayer of fear with his companions; the first prayer was obligatory, and he prayed with the second group as a voluntary prayer (peace and blessings be upon him) while they were praying it as an obligatory one. So there is no harm. If he finds them praying tarawih, he should pray with them with the intention of his obligatory prayer. Then, when the imam concludes the prayer, he should stand up and complete his prayer, and praise be to Allah. Yes, this is the correct ruling. Yes.