Musnad Abu Ya'la al-Mawsili 115:372

Ahmad narrated to us, Bahz narrated to us, Shu'bah narrated to us, Qatadah narrated to us, from Anas bin Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Straighten your rows, for the straightening of the row is part of the perfection of the prayer."