Musannaf Ibn Abi Shayba 3:1148

Abu al-Ahwas narrated to us, from Simak, from al-Nu'man ibn Bashir, who said: I have seen the Prophet (may Allah bless him and grant him peace) straightening the rows as if they were arrows, until one day he saw a man's chest protruding from the row. He said: "You shall straighten your rows, or Allah will surely cause discord between your faces."