Musnad Ahmad 32:790

Muhammad ibn Ja'far and Muhammad ibn Bakr narrated to us, they said: Sa'id and al-Khaffaf narrated to us, from Sa'id, from Qatadah, from Anas, that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Complete the bowing and the prostration, for by Allah, I surely see you from behind my back when you bow and when you prostrate."