Musnad Ahmad 652:95

Muhammad ibn Ja'far and Hajjaj narrated to us, they said: Shu'bah narrated to us, from Simak ibn Harb, he said: I heard al-Nu'man ibn Bashir saying: The Messenger of Allah (peace and blessings of Allah be upon him) used to straighten the row until he made it like an arrow or a gambling arrow. He said: Then he saw the chest of a man protruding, so the Messenger of Allah (peace and blessings of Allah be upon him) said: "O servants of Allah, you shall straighten your rows, or Allah will cause disagreement between your faces."