Musannaf Ibn Abi Shayba 3:156

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from al-Jurayri, from Abu Nadra, from Ka’b, who said: When you bow, direct your face toward the Qibla and place your hands upon your knees, and do not "tadhbah" (lower your head excessively) as the donkey "tadhbah."