Sahih Muslim Book - 4, Hadith - 979

حَدَّثَنَا يَحْيَى بْنُ يَحْيَى أَخْبَرَنَا أَبُو خَيْثَمَةَ عَنْ سِمَاكِ بْنِ حَرْبٍ قَالَ سَمِعْتُ النُّعْمَانَ بْنَ بَشِيرٍ يَقُولُ كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُسَوِّي صُفُوفَنَا حَتَّى كَأَنَّمَا يُسَوِّي بِهَا الْقِدَاحَ حَتَّى رَأَى أَنَّا قَدْ عَقَلْنَا عَنْهُ ثُمَّ خَرَجَ يَوْمًا فَقَامَ حَتَّى كَادَ يُكَبِّرُ فَرَأَى رَجُلًا بَادِيًا صَدْرُهُ مِنْ الصَّفِّ فَقَالَ عِبَادَ اللَّهِ لَتُسَوُّنَّ صُفُوفَكُمْ أَوْ لَيُخَالِفَنَّ اللَّهُ بَيْنَ وُجُوهِكُمْ.

An-Nu'man bin Bashir said: "The Messenger of Allah (s.a.w) used to straighten our rows, as if he was straightening an arrow, until he saw that we had learned it. Then he came out one day and was about to say the Takbir, when he noticed a man whose chest was sticking out from the row. He said: 'Slaves of Allah! Make your rows straight or Allah will cause discord among you."'