(76) باب إِلْزَاقِ الْمَنْكِبِ بِالْمَنْكِبِ وَالْقَدَمِ بِالْقَدَمِ فِي الصَّفِّ
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."
Reference: Sahih al-Bukhari 725
In-book reference: Book 10, Hadith 119
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 692