A man prayed alone behind the rows in the first rak'ah; is his prayer valid?

Question:
This is another questioner inquiring about a set of questions. In his first question, he says: A man entered the mosque while the imam was in the second rak'ah of the midday prayer, and he did not find a space in the row, so he prayed alone. In the following rak'ah, some people came and completed the row. Is the rak'ah that this man prayed alone valid, or must he repeat only that rak'ah? May you be rewarded.

Answer:
His prayer is not valid; because of the saying of the Prophet (peace and blessings be upon him): So if he prayed one rak'ah or more, his prayer is not valid, unless he invalidated it and started with the takbirat al-ihram when someone came with him, in which case it is valid. What he catches after that—meaning, if someone came with him in the last rak'ah, and he broke off his prayer and started with the takbirat al-ihram intending to pray with the imam when another person came with him—he would have caught a rak'ah, according to what happened to him. But if he built upon his prayer in which he was alone, his prayer is not valid; because of his saying (peace and blessings be upon him): And he saw a man praying alone behind the row, so he ordered him to repeat it—(peace and blessings be upon him).
The point is: prayer behind the row is invalid, and its performer must repeat it if it is an obligatory prayer.

The presenter: May Allah reward you.