Sahih al-Bukhari 718
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · Straightening the rows at the time of Iqama and after it (immediately)
Narrated Anas:
حَدَّثَنَا أَبُو مَعْمَرٍ ، قَالَ حَدَّثَنَا عَبْدُ الْوَارِثِ ، عَنْ عَبْدِ الْعَزِيزِ ، عَنْ أَنَسٍ ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " أَقِيمُوا الصُّفُوفَ فَإِنِّي أَرَاكُمْ خَلْفَ ظَهْرِي " .
The Prophet (ﷺ) said, "Straighten your rows, for I see you from behind my back.'