Musnad Ahmad 652:54

Husayn ibn Ali narrated to us, from Za'idah, from Simak, 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, until it was as if he were aligning arrows with us. Then, when he intended to say the takbir, he saw a man whose chest was protruding. He said: "You shall straighten your rows, or Allah will surely cause discord between your faces."