Musnad Abu Ya'la al-Mawsili 115:349

Ubayd Allah ibn Umar al-Qawariri narrated to us, Khalid narrated to us, Sa'id narrated to us, from Qatadah, from Anas, from the Prophet (peace and blessings of Allah be upon him), who said: "Complete the bowing and the prostration, for by Allah, I surely see you from behind my back when you bow or prostrate."