The Prophet ﷺ said: Complete the front row, then the one that comes next, and if there is any incompleteness, let it be in the last row. (Using translation from Abū Dāʾūd 671)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ أَتِمُّوا الصَّفَّ الْمُقَدَّمَ ثُمَّ الَّذِي يَلِيهِ فَمَا كَانَ مِنْ نَقْصٍ فَلْيَكُنْ فِي الصَّفِّ الْمُؤَخَّرِ