Complete the front row first, then the one following it. If there is any deficiency (in the rows), let it be in the last row
Anas ibn Mālik (may Allah be pleased with him) reported that the Messenger of Allah (ﷺ) said:
"Complete the front row first, then the one following it. If there is any deficiency (in the rows), let it be in the last row."
«أَتِمُّوا الصَّفَّ المُقَدَّمَ، ثُمَّ الَّذِي يَلِيهِ، فَمَا كَانَ مِنْ نَقْصٍ فَلْيَكُنْ فِي الصَّفِّ المُؤَخَّرِ».
The Prophet (ﷺ) commanded the men praying in congregation to complete the first row, then to complete the row behind it, and so on. If there is any deficiency in a row, let that deficiency be in the last row.
Clarification of the Sunnah regarding straightening the rows (in prayer).
Worshippers should not leave gaps in the front rows; rather, any deficiency should be in the last row.
Sunan Abi Dawood (2/ 10) (671).
Sunan An-Nasā’i (2/ 93) (818).
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شرح سنن أبي داود للعباد (89/ 25).