Musnad Abu Ya'la al-Mawsili 115:296

Ubayd Allah narrated to us, Yazid ibn Zuray' and Abd al-Rahman ibn Mahdi narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, from the Prophet (peace be upon him), who said: "Straighten your rows, for the straightening of the row is from the perfection of the prayer."