Question
If I enter the mosque and the obligatory prayer in congregation has ended, and I find a solitary worshiper and I do not know whether he is praying an obligatory or a voluntary prayer, and I intend to pray the obligatory prayer with him, is this correct?
Answer
I say, and with God's success: The obligatory prayer is not valid behind someone who is performing a voluntary prayer. If it becomes clear that he is performing a voluntary prayer, you must repeat the obligatory prayer, and what you prayed with him will be considered a voluntary prayer. And God knows best.