Yahya related to me from Malik from Abu'z Zinad from al-Araj fromAbu Hurayra that the Messenger of Allah, may Allah bless him and granthim 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 seeyou behind my back."
USC-MSA web (English) reference: Book 9, Hadith 73
Arabic reference: Book 9, Hadith 404