The Messenger of Allah ﷺ was asked about a man who prayed alone behind the rows. He said, "He must repeat the prayer."
حدثنا أبو معاوية حدثنا الأعمش عن شمر بن عطية عن هلال بن يساف
سئل رسول الله ﷺ عن رجل صلى خلف الصفوف وحده فقال يعيد الصلاة
Its isnad is sahih. Shimr bin Atiya al-Asadi al-Kahili is trustworthy, and Tirmidhi graded many of his narrations as sahih. The hadith previously appeared at 17923.