Mukhtasar Sahih Bukhari — Hadith 267

Narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: "You think I cannot see you from the front, but by Allah, neither your humility nor your bowing is hidden from me, and indeed I see you from behind my back (as well)."