Musnad Ahmad 652:43

Yahya ibn Sa'id narrated to us, from Shu'bah, 'Amr ibn Murrah narrated to me, he said: I heard Salim ibn Abi al-Ja'd, he said: I heard al-Nu'man ibn Bashir, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "You shall straighten your rows, or Allah will surely cause disagreement between your faces."