Is the action of a worshiper who moves backward to stand in line with a man praying alone in the row considered valid, or should he have left him to pray alone, knowing that the prayer of a person praying alone behind the row is not accepted?

If a worshiper does not find a space in the row, he may alert someone verbally or otherwise to stand with him. Sheikh Al-Uthaymeen held the view that the prayer of one who prays alone behind the row is valid if there is an excuse for it, citing the Almighty's saying: ﴿So fear Allah as much as you are able﴾ [At-Taghabun: 16], and His saying: ﴿Allah does not burden a soul beyond its capacity﴾ [Al-Baqarah: 286]. It is not permissible for a worshiper to pull someone from the row to pray with him, as this causes disturbance and breaks the row. As for the questioner moving backward to complete the row, this does not invalidate the prayer, although it is disliked without need.