Sahih Muslim 436 b
Nu'man b. Bashir reported:
The Messenger of Allah (may peace-be upon him) used to straighten our rows as it lie were straightening an arrow with their help until be saw that we had learnt it from him. One day he came out, stood up (for prayer) and was about to say: Allah is the Greatest, when he saw a man, whose chest was bulging out from the row, so he said: Servants of Allah, you hint straighten your rows or Allah would create dissension amongst you.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا أَبُو خَيْثَمَةَ، عَنْ سِمَاكِ بْنِ حَرْبٍ، قَالَ سَمِعْتُ النُّعْمَانَ بْنَ بَشِيرٍ، يَقُولُ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُسَوِّي صُفُوفَنَا حَتَّى كَأَنَّمَا يُسَوِّي بِهَا الْقِدَاحَ حَتَّى رَأَى أَنَّا قَدْ عَقَلْنَا عَنْهُ ثُمَّ خَرَجَ يَوْمًا فَقَامَ حَتَّى كَادَ يُكَبِّرُ فَرَأَى رَجُلاً بَادِيًا صَدْرُهُ مِنَ الصَّفِّ فَقَالَ ‏
"‏ عِبَادَ اللَّهِ لَتُسَوُّنَّ صُفُوفَكُمْ أَوْ لَيُخَالِفَنَّ اللَّهُ بَيْنَ وُجُوهِكُمْ ‏"‏ ‏.‏