Musnad Ahmad 32:1955

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