What is the ruling on an individual praying alone behind the imam at the back of the mosque, if the mosque does not adhere to social distancing in the circumstances of the coronavirus pandemic?

The jurists differed regarding the prayer of a person praying alone behind a row. The majority of scholars held the view that it is disliked (makruh (definition)), yet valid despite the dislike, citing the hadith (definition) of Abu Bakra. Whereas the Hanbalis held the view that it is invalid if one prays alone for an entire rak'ah without an excuse, citing the hadith of Wabisa ibn Ma'bad and Ali ibn Shayban.

Shaykh al-Islam Ibn Taymiyyah and a group of scholars, including Shaykh Ibn 'Uthaymeen, held the view that the prayer is valid and there is no sin if the solitary prayer is due to an excuse, such as the absence of a place in the row. This is because obligations are waived due to inability and necessity. So, if a worshiper does not find a place in the row, he prays alone behind the row, and his prayer is valid. The obligation of joining the row is waived for him due to the excuse, just as other prayer obligations are waived due to necessity. This also applies to someone who fears illness and prays alone behind the row; his prayer is valid because he abandoned the obligation due to an excuse.