It was narrated that An-Numan bin Bashir said:"The Messenger of Allah (ﷺ) used to straighten the rows like shaft of an arrow is straightened before the head is attached to it. He saw a man whose chest was sticking out from the row. I saw the Messenger of Allah (ﷺ) say: 'Make your rows straight or Allah will cause your faces to be deformed."
أَخْبَرَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ أَنْبَأَنَا أَبُو الأَحْوَصِ، عَنْ سِمَاكٍ، عَنِ النُّعْمَانِ بْنِ بَشِيرٍ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُقَوِّمُ الصُّفُوفَ كَمَا تُقَوَّمُ الْقِدَاحُ فَأَبْصَرَ رَجُلاً خَارِجًا صَدْرُهُ مِنَ الصَّفِّ فَلَقَدْ رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يَقُولُ " لَتُقِيمُنَّ صُفُوفَكُمْ أَوْ لَيُخَالِفَنَّ اللَّهُ بَيْنَ وُجُوهِكُمْ " .
Nesai
The Book of Leading the Prayer (Al-Imamah) • Hadith 810