The Book of Leading the Prayer (Al-Imamah)
10/28. Chapter:
The Imam encouraging (worshippers) to make the rows solid and stand close to one another
Sunan an-Nasai
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 " .
It was narrated that Anas said: "The Messenger of Allah (ﷺ)turned 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."'
Reference : Sunan an-Nasa'i 814In-book reference : Book 10, Hadith 38English translation : Vol. 1, Book 10, Hadith 815