“The Messenger of Allah ﷺ used to straighten the rows until he made them like a spear or an arrow-shaft. Once he saw a man’s chest (sticking out) so the Messenger of Allah ﷺ said: ‘Make your rows straight or Allah will create division among you.’” (Using translation from Ibn Mājah 994)
سَمِعْتُ النُّعْمَانَ بْنَ بَشِيرٍ يَقُولُ كَانَ رَسُولُ اللهِ ﷺ يُسَوِّي الصَّفَّ حَتَّى يَجْعَلَهُ مِثْلَ الرُّمْحِ أَوِ الْقَدَحِ قَالَ فَرَأَى صَدْرَ رَجُلٍ نَاتِئًا فَقَالَ رَسُولُ اللهِ ﷺ يَا عِبَادَ اللهِ لَتُسَوُّنَّ صُفُوفَكُمْ أَوْ لَيُخَالِفَنَّ اللهُ بَيْنَ وُجُوهِكُمْ