Sahih al-Bukhari 718
Book 10: Call to Prayers (Adhaan) · Narrated by Anas
Arabic
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 ".
English translation
Narrated Anas:The Prophet (ﷺ) said, "Straighten your rows, for I see you from behind my back
Reference: Sahih al-Bukhari 718, Book of Call to Prayers (Adhaan). English translation by Dr. Muhammad Muhsin Khan.