Musnad Ahmad 652:84

Waki’ narrated to us, Zakariya narrated to us, from Abu al-Qasim al-Jadali, who said: My father said: And Yazid ibn Harun narrated to us, Zakariya informed us, from Husayn ibn al-Harith Abu al-Qasim, that he heard al-Nu’man ibn Bashir say: The Messenger of Allah (may Allah bless him and grant him peace) turned his face toward the people and said: "Straighten your rows," three times. "By Allah, you shall either straighten your rows, or Allah will cause discord between your hearts." He said: "Then I saw a man joining his heel to the heel of his companion, his knee to his knee, and his shoulder to his shoulder."