Simak binHarb narrated that he heard Nu'man bin Bashir say:
"TheMessengerof Allah (ﷺ) used to straighten the rows until he madethem like aspear or an arrow-shaft. Once he saw a man's chest(sticking out)so the Messenger of Allah (ﷺ) said: 'Make your rowsstraight orAllah will create division among you.'" (FR)
English reference (FR): Vol. 1, Book 5, Hadith 994 (FR)
Arabic reference (FR): Book 5, Hadith 1047 (FR)