Musannaf Ibn Abi Shayba 3:211

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 'dabh' (stretch/droop) as the donkey 'dabhs'."