Musnad Abu Ya'la al-Mawsili 115:130

Abu Khaythama narrated to us, Yahya narrated to us, from Shu'bah, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Complete the bowing (ruku') and the prostration (sujud), for by Allah, I surely see you from behind my back."