Musnad Abu Ya'la al-Mawsili 129:4

Abu Khaythama narrated to us, Hushaym narrated to us, Humayd al-Tawil informed us, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Straighten your rows and stand close together, for indeed I see you from behind my back."