Musnad Ahmad 652:89

Waki’ narrated to us, from Sufyan, from Simak, from al-Nu’man ibn Bashir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in prayer, and he saw a man whose chest was protruding from the row. He said: "Align yourselves, and do not differ, lest your hearts differ."