Sunan an-Nasa'i 818
سنن النسائي#818Sahihأَخْبَرَنَا 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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 10:42