This view is very sound, and it is stronger than the view that his prayer is invalid if he stands to the imam’s left while the imam’s right side is unoccupied. Declaring a person sinful, or declaring his prayer invalid, without evidence that gives the heart confidence is open to question. Invalidating an act of worship without a textual proof is like validating it without a textual proof.

“Nor [is the prayer valid] for one who prays alone behind him or behind the row …”

His statement, “Nor for one who prays alone behind him,” means that the prayer of a single follower standing behind the imam is invalid. As for the imam, there is some detail: if he remains upon the intention of leading the prayer, his prayer is invalid, because he intended to be an imam while no one is praying with him. But if he intends to pray alone, his prayer is valid.

His statement, “or behind the row,” means that the follower’s prayer behind the row is invalid, because he is praying alone. A hadith from the Messenger of Allah ﷺ states: “There is no prayer for one who prays alone behind the row.” (1) The Prophet ﷺ saw a man praying alone behind the row and instructed him to repeat the prayer. (2) Had it not been invalid, he would not have instructed him to repeat it, for requiring its repetition is an obligation and a burden imposed regarding something that has already been performed and completed. Had that completed act not been invalid, a person would not have been required to repeat it, since this would entail that the act of worship be obligatory upon him twice.

What the author, may Allah have mercy on him, stated is the established position of the school, and it is one of its distinctive rulings.