The ruling on entering the prayer behind a person who prayed alone

Question:
If a person enters and the prayer has already ended, and he prays, and then other people enter and pray behind him, what is the ruling on that?

Answer:
If this person entered the state of ihram alone and others came to pray with him, there is no harm in that, provided he entered the prayer alone and others came and prayed with him for the sake of congregation. This is permissible, because Ibn Abbas (may Allah be pleased with him) said: He got up from his bed when he saw the Prophet (peace and blessings be upon him) getting up to perform the night prayer. He got up from his bed, performed ablution, and came and stood to the left of the Messenger (peace and blessings be upon him). He came to the left of the Messenger (peace and blessings be upon him), so the Messenger (peace and blessings be upon him) moved him to his right. This indicates that it is permissible for a person to join one who is praying alone, whether it is an obligatory prayer or a voluntary one. That is, if it is permissible in a voluntary prayer, it is permissible in an obligatory prayer, unless there is evidence indicating a difference. However, if a person is late to part of the prayer, and then stands up after the imam has finished to make up what he missed or what he was late for, this is the matter that is subject to scrutiny. One who comes in should not join him, because he entered the prayer as a follower, so he does not become an imam during the prayer. This is a matter of scrutiny among the scholars. But if someone did it and prayed with him, it would be in the clear prayer. However, from the beginning, one should say: do not join him, because this is a matter of doubt and a matter of suspicion. Yes.