Musnad Ahmad 652:81

Bahz narrated to us, Hammad ibn Salamah narrated to us, Simak ibn Harb informed us, from al-Nu’man ibn Bashir, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to straighten us in the rows, just as arrows are straightened, until he thought that we had learned that from him and understood it. One day, he turned his face, and there was a man whose chest was protruding [out of the line]. He said: "You shall straighten your rows, or Allah will surely cause discord between your faces."