Musannaf Ibn Abi Shayba 3:1147

Abu Bakr narrated to us, he said: Hushaym ibn Bashir narrated to us, he said: Humayd informed us, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Straighten your rows, 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. And if you were to attempt to do that today, you would see one of them as if he were a restive mule."