What is the ruling on congregational prayer in a mosque where the worshippers leave a large gap between their rows, and does their prayer become invalidated by that?

It is sunnah for worshippers to straighten their rows, close the gaps between them, and align their necks, as narrated by Anas (may Allah be pleased with him) that the Prophet (peace be upon him) said: "Straighten your rows, close the gaps between them, and align your necks." Al-Sindi and Al-Munawi indicated that closing the gaps between rows means minimizing the distance between rows so that another row cannot fit between any two rows. Leaving the front rows and praying at the back of the mosque is contrary to the Sunnah and causes disputes. As for the validity of their prayer, their prayer is valid as long as they are praying inside the mosque and standing in a row, and none of them stands alone behind the rows. Similarly, praying in the mosque's courtyard is valid, even if it is contrary to the Sunnah, provided the worshippers are inside. It is obligatory to continue advising them and urging them to follow the Sunnah.