Do not pray behind the rows alone

Q: From 'Abd al-'Aziz ibn 'Abd Allah ibn Baz to the honorable brother A. A. 'A. K., may Allah guide him.
Peace be upon you, and Allah’s mercy and blessings. After that:
With reference to your inquiry, registered with the Research Department under number 1221 on 23/4/1405 AH, we inform you that it has been reviewed, and here is the answer to each question, followed by its response:
Q: A man entered the mosque and found the prayer in progress and the rows full, and he did not find a gap, so he prayed behind the congregation, behind the rows, and none of the worshippers pulled him forward. Is his prayer valid or not?

A: His prayer is not valid according to the more correct of the two opinions of the scholars, due to the saying of the Prophet (peace and blessings be upon him): And because he (peace and blessings be upon him) saw a man praying alone behind the rows and ordered him to repeat the prayer. It was narrated by Ahmad, Abu Dawud, and al-Tirmidhi, and Ibn Hibban graded it as authentic, and its chain is good. And success is from Allah, and may Allah send prayers and blessings upon our Prophet Muhammad, his family, and his Companions.
The General President of the Departments of Research
Scientific, Ifta, Da'wah, and Guidance