Musnad Ahmad 32:381

Muhammad ibn Ja'far narrated to us, he said: Shu'bah narrated to us, and Yazid said: Shu'bah informed us, from Qatadah. Ibn Ja'far said in his narration: I heard Qatadah narrating from Anas, from the Prophet, may Allah bless him and grant him peace, that he said: "Complete the bowing and the prostration, for by Allah, I surely see you from behind me—and he sometimes said: from behind my back—when you bow and when you prostrate."