Maliks Muwatta Book 9, Hadith Number 73.
Section : How to Do the Prayer in General.
Yahya related to me from Malik from Abu’z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “Do you see the direction I am facing here? By Allah, neither your concentration nor your ruku is hidden from me. I can see you behind my back.”