What is the ruling on the prayer of a person praying alone if the rows are complete?

The questioner : In the name of Allah, the Most Gracious, the Most Merciful.
His EminenceThe Shaykh : I entered a mosque and found all the rows full. After I finished my prayer, the imam told me: "Repeat the prayer, because you prayed behind the rows alone." What is the validity of this statement?

The Shaykh : If a person comes and finds the rows complete, he should pray behind the rows alone with the imam, and he does not have to repeat it. This is the stronger opinion among the opinions of the scholars. We are aware that there are three opinions of the scholars regarding this issue. The first opinion is that the prayer of a person praying alone behind the rows is valid, even if he has a place in the rows in front of him, and that joining the row is recommended, not obligatory. This is the school of the majority of the Muslim community, because it is the school of Imam Malik, Imam al-Shafi'i, Imam Abu Hanifah, and a narration from Imam Ahmad. That is, it means the text of Imam Ahmad's school with those who permit a row behind a row, even if one has a place. Do you understand now? Good.
The second opinion is that if a person prays alone behind the row, even if he did not find a place, his prayer is not valid. And if you compare the two opinions, you will find that this is the extreme of strictness and that one is the extreme of ease.
The third opinion is the middle one: that if you find the row complete, you stand alone, because Allah says: And I do not have the ability. So if a person tells you, "You have the ability, pull one person in," we say: this is not permissible, because pulling one person in is an aggression upon him, and it causes disturbance to the rest of the worshippers, and it breaks the row. As for it being an aggression upon the one you pulled, it is because you are moving him from a superior place to a place that is what? Inferior, and you also disturb his prayer, because he does not know what pulled him. As for it being a disturbance to the worshippers, it is because this row will all come closer, they will move without any need in order to fill the gap in this row. And as for it being a breaking of the row, that is clear. If someone says: this person can move forward and stand with the imam, we say: this is also wrong, because if he moves forward, he will harm the worshippers in front of him by weaving through them. And if the mosque has twenty rows, how many does he weave through? Yes? Twenty rows, so he harms them. Then, if he moves forward and stands with the imam, and another person comes after him a minute or two later, what do we say? Yes? Go with the imam. Now there are two with the imam. And after a minute or two, the third comes, we say: go with the imam. And the imam has a complete row. But if he stands in the row and someone comes to him, they become a row. And if no one comes to him, his being with the congregation in the actions, even if he is not with them in the row, is better than his not being with them in the row or in the actions. Therefore, this middle opinion is the choice of Shaykh al-Islam Ibn Taymiyyah, and the choice of our Shaykh Abd al-Rahman ibn Sa'di (may Allah have mercy on him), that whenever a person finds the rows complete in front of him, he should join the imam, even if he prays alone, and his prayer is valid. But if you repeated the prayer based on the imam's statement, it is for the best that the second prayer be a voluntary prayer if the first was valid, or that the second be valid and the first be a voluntary prayer if the first was not valid. Not valid means an obligatory prayer. But you know the stronger opinion now: that if you come and the rows are complete, pray with the imam, even if you are alone. Yes.