Musnad Abu Ya'la al-Mawsili 115:156

Muhammad ibn al-Muthanna narrated to us, Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: And I heard Qatadah narrating from Anas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Straighten your rows, for the straightening of the row is from the perfection of the prayer."