Sahih al-Bukhari, Book 10, Hadith 119 (In-Book) ⓘ
Narrated Anas bin Malik: The Prophet 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.
also referenced under: Hadith 725 (DarusSalam) & Volume 1, Book 11, Hadith 692 (USC-MSA)