Is it obligatory for one who prayed a single rak'ah behind the row alone to repeat the prayer once the row was completed?

Question:
This questioner, Abu Mu'adh from Riyadh, says: Your Eminence, the Shaykh, our father, 'Abd al-'Aziz ibn Baz (may Allah preserve him), I have a set of questions. In the first question: What is the correct view among the scholars regarding the prayer of one who is alone behind the row? And is the prayer of one who prayed a single rak'ah behind the row alone, and then the row was completed afterwards, valid, or is it obligatory for him to repeat it?

Answer:
Whoever prays behind the row must repeat the prayer, based on the saying of the Prophet (peace and blessings be upon him): However, if he arrived while the imam was bowing, and he bowed alone, and then another person joined him before prostration, the prayer is valid.
As for if he prayed a rak'ah alone, the prayer is not valid unless he restarts it after that rak'ah. He restarts the takbir regarding the remainder, meaning: he restarts his entry with his imam after the rak'ah in which he was alone. He considers it invalid and performs the takbir of intention in the remaining rak'ahs; then it is valid, and he makes up what he missed.
The point is: the prayer of one who is alone behind the row is not valid absolutely, whether he finds someone to stand in the row with him or not. He does not pray behind the row, but rather he waits until someone comes to stand in the row with him, or he looks for a gap, or he stands in the row with the imam on his right if that is feasible. This is what is obligatory. However, merely that he said the takbir alone, or bowed alone, and then someone came and stood in the row with him; this is valid, yes.
The presenter: May Allah reward you.