Action when two followers are behind the Imam and one leaves
Chapter on Imamate and Congregational Prayer
Al-Mughni
Book of Prayer
Primary text
If two people begin praying behind the Imam, and one leaves due to an excuse or otherwise, the other should join the row, or he should notify someone else to come and stand with him. If none of this is possible, he should intend to pray alone and complete it individually, as an unforeseen circumstance has occurred, similar to when the Imam loses his ablution.