The Book of Leading the Prayer (Al-Imamah)
10/30. Chapter:
The last row
Sunan an-Nasai
أَخْبَرَنَا إِسْمَاعِيلُ بْنُ مَسْعُودٍ، عَنْ خَالِدٍ، قَالَ حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " أَتِمُّوا الصَّفَّ الأَوَّلَ ثُمَّ الَّذِي يَلِيهِ وَإِنْ كَانَ نَقْصٌ فَلْيَكُنْ فِي الصَّفِّ الْمُؤَخَّرِ " .
It was narrated from Anas that the Messenger of Allah (ﷺ) said: "Complete the first row, then the one behind it, and if any row is to be left incomplete let it be the last row."
Reference : Sunan an-Nasa'i 818In-book reference : Book 10, Hadith 42English translation : Vol. 1, Book 10, Hadith 819