Sahih al-Bukhari 725
Narrated Anas bin Malik:
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."
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.‏