Musnad Abu Ya'la al-Mawsili 115:373

Ahmad narrated to us, Abu Dawud narrated to us, Shu'bah narrated to us, from Qatadah, who said: Anas said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Straighten your rows, for the straightening of the row is part of the establishment of prayer." Abu Dawud said: Shu'bah said: I was hesitant regarding this; I did not ask Qatadah whether he heard it [directly] or not.