Sunan an-Nasa'i, Hadith 845 ⓘ
It was narrated that Anas said: "The Messenger of Allah turned to face us when he stood up to pray, before he said Takbir, and said: 'Make your rows straight and fill the gaps, for I can see you from behind my back."'
أَنْبَأَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا إِسْمَاعِيلُ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ، قَالَ أَقْبَلَ عَلَيْنَا رَسُولُ اللَّهِ صلى الله عليه وسلم بِوَجْهِهِ حِينَ قَامَ إِلَى الصَّلاَةِ قَبْلَ أَنْ يُكَبِّرَ فَقَالَ " أَقِيمُوا صُفُوفَكُمْ وَتَرَاصُّوا فَإِنِّي أَرَاكُمْ مِنْ وَرَاءِ ظَهْرِي " .
also referenced under: Book 10, Hadith 69 (In-Book) & Volume 1, Book 10, Hadith 846 (DarusSalam)