Musnad Ahmad 30:1756

Ishaq ibn Isa narrated to us, Malik informed us, from Abu al-Zinad, from al-A'raj, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do you see my qiblah here? By Allah, your khushu' (humility/devotion) and your ruku' (bowing) are not hidden from me. Indeed, I see you from behind my back."