Sunan an-Nasa'i, Hadith 814 ⓘ
It was narrated that Anas said: "The Messenger of Allahturned to face us when he stood up to pray, before he said the Takbir and said: 'Make your rows straight and come close to one another, for I can see you behind my back."'
أَخْبَرَنَا عَلِيُّ بْنُ حُجْرٍ، أَنْبَأَنَا إِسْمَاعِيلُ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ، - رضى الله عنه - 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 38 (In-Book) & Volume 1, Book 10, Hadith 815 (DarusSalam)