Musnad Abu Ya'la al-Mawsili 115:316

Abu Musa narrated to us, Ibn Abi Adi narrated to us, from Sa'id, from Qatadah, from Anas, that the Prophet of Allah (peace 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." Then Qatadah said: "Allah shows him of that what you do not see."