Where Should the Third Person Stand When Joining Congregational Prayer?
Source: Aḥkām wa Masāʾil – Bāb al-Ṣalāh, Vol. 1, p. 169
Question:
If two people are already offering prayer in congregation, how should a third person join them, and on which side should he stand?
Answer:
Al-ḥamdu lillāh, waṣ-ṣalātu wa-s-salāmu ʿalā Rasūlillāh, ammā baʿd!
If two people are offering prayer in congregation and a third person arrives, the Imām should move the existing follower behind him, so that both followers form a row behind the Imām, and the congregation is completed in the proper manner.
This is established in Ṣaḥīḥ Muslim:
In Ṣaḥīḥ Muslim (Vol. 2, p. 317), it is narrated that Jābir ibn ʿAbdillāh رضي الله عنهما was praying with the Messenger of Allah ﷺ. Meanwhile, Jabbār ibn Ṣakhr رضي الله عنه came and stood on the left side of the Prophet ﷺ. The Messenger of Allah ﷺ then moved both of them behind him.
This action proves that when a third person joins the congregation, the Imām should place the followers behind him, so that he stands in front and the rest form a row behind him.
Hādhā mā ʿindī, wallāhu aʿlam bi-ṣ-ṣawāb.