Musnad Ahmad 32:1720

'Affan narrated to us, Shu'bah narrated to us, he said: Qatadah informed me, from Anas ibn Malik, who said: "Straighten your rows, for the straightening of the row is part of the perfection of the prayer." 'Abdullah said: "I think he attributed it to the Prophet, may Allah bless him and grant him peace, and I suspect that I may have omitted it."