Volume 8 · Book 78 · Hadith 639
Narrated Anas bin Malik: I heard the Prophet saying, "Perform the bowing and the prostration properly (with peace of mind), for, by Him in Whose Hand my soul is, I see you from behind my back when you bow and when you prostrate."
— Sahih Bukhari, Volume 8, Book 78: Oaths and Vows, Hadith #639