The ruling on the prayer of one who prays alone behind the rows

Question:
The questioner, Abu 'Abdullah, asks: What is the ruling on the prayer of a person who is alone behind the rows? If he arrives and the first row is already complete, and he remains alone and prays until the end, and the prayer concludes?

Answer:
His prayer is invalid, because the Prophet (peace and blessings be upon him) saw a man praying alone behind the rows and ordered him to repeat it, and he said in the hadith: The one who is alone is the one who is separated. So if he arrives and the row is already complete, he should either try to enter the row if possible, or enter to the right of the imam. Otherwise, he waits until someone comes to pray with him. Yes. And if he wants to catch up with the congregation and the reward of catching up with the congregation, what he catches up with in this case—meaning, being alone behind the row—his prayer is not valid. Yes.

The presenter:
May Allah reward you,