Narrated Abu Huraira
Allah's Messenger (ﷺ) said, "You see me facing the Qibla; but, by Allah, nothing is hidden from me regarding your bowing and submissiveness and I see you from behind my back."
Narrated Anas bin Malik
The Prophet (ﷺ) said, "Perform the bowing and the prostrations properly. By Allah, I see you from behind me (or from behind my back) when you bow or prostrate."