Musnad Ahmad 32:1834

Mu'awiyah narrated to us, Za'idah narrated to us, Humayd al-Tawil narrated to us, Anas ibn Malik narrated to us: He said: The prayer was called, and the Messenger of Allah, peace and blessings of Allah be upon him, turned to us with his face and said: "Straighten your rows and stand close together, for I see you from behind my back."