Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 855
Anas b. Malik reported. The Messenger of Allah (may peace be upon him) said: Perform bowing and prostration well. By Allah. I see you even if you are behind me, or he said'. (1 see you) behind my back when you bow or prostrate.