The ruling on one who prays alone behind the row for one rak'ah or more

Question:
If a person enters the mosque and the rows are already formed, and he stands alone behind the row, says the takbir, and bows for two rak'ahs or one, and then people come and stand with him in the same row?

Answer:
He should restart the prayer. If they come, he should restart from the beginning; he should say the takbir and begin, because his first prayer was not valid. His prayer alone is not valid. The Prophet (peace and blessings be upon him) said: , and when he saw a man praying behind the row, he ordered him to repeat the prayer (peace and blessings be upon him).
But if they come while he is in the first rak'ah before prostration, it suffices; due to the story of Abu Bakrah, where he came while they were in the first rak'ah after bowing or before bowing, and it sufficed. Otherwise, he must break off the prayer and start anew, if one or more people come with him so that they form a new independent congregation of two or more, or if he finds a gap in which he can enter.
Q: If he is ignorant of the ruling?
The Shaykh: Even if he is ignorant. The Prophet (peace and blessings be upon him) did not ask for details; he ordered the one praying alone behind the row, and did not say, "Do you have knowledge or do you not have knowledge?" This is to block the means, to block the means of laxity.
Q: Is the prayer of the ignorant person invalid?
The Shaykh: Yes, he must repeat it if he prayed alone behind the row.