Musnad Abu Ya'la al-Mawsili 117:17

Narrated to us Abd al-Rahman, narrated to us Hammad, from Thabit and Humayd, from Anas: That the Messenger of Allah (may Allah bless him and grant him peace) used to say: "Straighten yourselves," two or three times. "By the One in Whose hand is my soul, I see you from behind me just as I see you from in front of me." And Humayd added in the hadith: "Straighten yourselves and close your ranks."