Validity of standing to the Imam's left when a row is established behind the Imam
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
If the single follower stands to the Imam's left, and there is an established row behind the Imam, his prayer may be valid. This is based on the report that the Prophet (peace be upon him) sat to the left of Abu Bakr, who was leading. Also, since the prayer of those behind the Imam is validly formed, standing to the Imam's left is permitted, similar to having another person on the right.
Supporting text
It is held that the prayer might be invalid because the left side is not a valid position if there is no row behind, resembling the position in front of the Imam. The distinction from the case where there is someone on the right is that in that instance, they form one row, similar to standing behind the row together.