Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(76)
Chapter: To stand shoulder to shoulder and foot to foot in a row
(76)
باب إِلْزَاقِ الْمَنْكِبِ بِالْمَنْكِبِ وَالْقَدَمِ بِالْقَدَمِ فِي الصَّفِّ
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.‏
Reference : Sahih al-Bukhari 725
In-book reference : Book 10, Hadith 119
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 692
  (deprecated numbering scheme)