Guidance for one who enters the mosque after the congregation and finds a man praying

Question:
He says: If I enter the mosque and find someone praying, but I do not know whether he is praying the recommended practice or the obligatory prayer, what should I do?

Answer:
If the congregation has not yet arrived, you wait and pray the greeting prayer. If there is a regular voluntary prayer prescribed before it, such as for the midday prayer, you pray that regular voluntary prayer and wait for the congregation until they arrive, then you pray with them.
As for if they have already prayed, you pray with him so that you form a congregation; you are the follower and he is the imam, so that you obtain the virtue of the congregation. Even if it is a voluntary prayer, you obtain the virtue of the congregation. Or you ask and say: Have the people prayed? If there is doubt in the matter, you say: Have the people prayed? If they have not prayed, you wait. If the people have prayed, you pray with him so that you obtain the virtue of the congregation. Yes.

The presenter : May Allah reward you with good.