Musannaf Ibn Abi Shayba 3:325

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from Abdullah ibn Sa'id, from his grandfather, from Abu Hurayrah, who attributed it (to the Prophet), that he said: "When one of you prostrates, let him begin with his knees before his hands, and let him not kneel like the kneeling of a camel."