Musnad Ahmad 652:94

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, 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 in your prayer, or Allah will surely cause disagreement between your faces."