Musnad Abu Ya'la al-Mawsili 115:214

Zuhayr narrated to us, Abu ‘Amir narrated to us, Shu‘bah narrated to us, from Qatadah, from Anas, that the Prophet 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, or from the perfection, of the prayer."