Musnad Abu Ya'la al-Mawsili 129:141

Abu Khaythama narrated to us, Yazid narrated to us, Humayd informed us, from Anas bin Malik, that the Prophet (peace be upon him), after the prayer had been called and before he said the Takbir, turned his face toward the people and said: "Straighten your rows and stand close together, for I see you from behind my back." He (Anas) said: "I would see the man in the row sticking his shoulder to the shoulder of his companion."