Musannaf Ibn Abi Shayba 3:1152

Abu Usama narrated to us, from Sa'id, from Qatada, from Yunus ibn Jubayr, from Hattan ibn Abdullah al-Raqashi, who said: Abu Musa al-Ash'ari led us in prayer. When he turned away [from the prayer], he said: "The Prophet of Allah, may Allah bless him and grant him peace, delivered a sermon to us, in which he clarified our Sunnah for us and taught us our prayer. He said: 'When you pray, straighten your rows.'"