Musnad Ahmad 32:1951

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Qatadah narrating from Anas ibn Malik, from the Prophet, may Allah bless him and grant him peace, that he said: And Yahya ibn Sa'id narrated to us, from Shu'bah, Qatadah narrated to us, from Anas, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Complete the bowing and the prostration, for by Allah, I surely see you behind me - and sometimes he said: from behind my back - when you bow and when you prostrate."