Musnad Abu Ya'la al-Mawsili 129:3

Abu Bakr 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 yourselves in your prayer and stand close together, for I see you from behind my back." Anas said: "I have seen one of us sticking his shoulder to the shoulder of his companion and his foot to his foot. If you were to attempt to do that today, you would see one of them as if he were a skittish mule."