(10) The Book of Leading the Prayer (Al-Imamah)
كتاب الإمامة
(27) Chapter: Hoe many times should he say "Make your rows straight?"
(27) باب كَمْ مَرَّةٍ يَقُولُ اسْتَوُوا
Sunan an-Nasa'i 813 
It was narrated from Anas that the Prophet (ﷺ) used to say:
"Make your rows straight, make your rows straight, make your rows straight. By the One in Whose Hand is my soul! I can see you behind me as I can see you in front of me."
أَخْبَرَنَا أَبُو بَكْرِ بْنُ نَافِعٍ، قَالَ حَدَّثَنَا بَهْزُ بْنُ أَسَدٍ، قَالَ حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ ثَابِتٍ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يَقُولُ ‏ "‏ اسْتَوُوا اسْتَوُوا اسْتَوُوا فَوَالَّذِي نَفْسِي بِيَدِهِ إِنِّي لأَرَاكُمْ مِنْ خَلْفِي كَمَا أَرَاكُمْ مِنْ بَيْنِ يَدَىَّ ‏"‏ ‏.‏
Grade :  Sahih (Darussalam)
Reference : Sunan an-Nasa'i 813
In-book reference : Book 10, Hadith 37
English translation : Vol. 1, Book 10, Hadith 814