The ruling on the prayer of one who stands alone behind the rows and of one who prays with him

Question:
He has another question in which he says: I entered the mosque and found one of the ma'mum (those praying behind the imam) standing alone behind the rows. I stood with him and joined the prayer. Then the row lengthened, and the prayer ended. I said to him: "My brother, standing alone behind the rows invalidates the prayer of one who does so, as I have heard on this program." He told me that he had tried to pull someone from the row in front of him, but no one responded to him. Now, what is the validity of his prayer, and what is the validity of what I did?
May Allah reward you with good.

Answer:
If the person who preceded you stood in the row, and then you arrived before the bowing (ruku'), or after the bowing, then both of your prayers are valid. However, if you arrived after he had prayed one rak'ah or more, then your prayer is valid because you believed his prayer to be sound, but his prayer is not valid; he must repeat it.
As for if you knew that his prayer was invalid when you stood with him, then you must also repeat it. But if you did not believe that and were unaware of the legal ruling, then your prayer is valid. Or, if he stood before the bowing, or after the bowing, but you joined him before he prostrated in the first rak'ah, then both of your prayers are valid as well, due to the incident of Abu Bakrah. Abu Bakrah al-Thaqafi (may Allah be pleased with him) came while the Prophet (peace and blessings be upon him) was bowing, so he bowed behind the row, then entered the row. The Prophet (peace and blessings be upon him) said to him: and did not order him to repeat the prayer. This indicates that if one stands behind the row during the bowing, or after the bowing, but catches the prostration in the row, or stands in the row with others, then his prayer is valid. Yes.

The presenter: May Allah reward you with good.