Musnad Abu Ya'la al-Mawsili 139:24

'Abd al-A'la ibn Hammad narrated to us, Mu'tamir narrated to us, he said: I heard my father, that a man narrated to him from Anas ibn Malik that he said: "One of us would keep his back straight in prayer behind the Prophet, may Allah bless him and grant him peace, until the Prophet, may Allah bless him and grant him peace, was firmly established in prostration—or he said: [firmly established] on the ground—then he would prostrate after that."