Sahih al-Bukhari 725
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · To stand shoulder to shoulder and foot to foot in a row
Narrated Anas bin Malik:
حَدَّثَنَا عَمْرُو بْنُ خَالِدٍ ، قَالَ حَدَّثَنَا زُهَيْرٌ ، عَنْ حُمَيْدٍ ، عَنْ أَنَسٍ ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " أَقِيمُوا صُفُوفَكُمْ فَإِنِّي أَرَاكُمْ مِنْ وَرَاءِ ظَهْرِي ". وَكَانَ أَحَدُنَا يُلْزِقُ مَنْكِبَهُ بِمَنْكِبِ صَاحِبِهِ وَقَدَمَهُ بِقَدَمِهِ.
The Prophet (ﷺ) said, "Straighten your rows for I see you from behind my back." Anas added, "Everyone of us used to put his shoulder with the shoulder of his companion and his foot with the foot of his companion."