Musnad Ahmad 30:905

I read to Abd al-Rahman: Malik, 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, neither your khushu' (humility/devotion) nor your ruku' (bowing) is hidden from me; indeed, I see you from behind my back."