Musnad Ahmad 25:1074

Hashim narrated to us, from Ibn Abi Dhi'b, from Shu'bah, who said: A man came to Ibn Abbas and said: "Your freedman, when he prostrates, places his forehead, his forearms, and his chest upon the ground." Ibn Abbas said to him: "What compels you to do what you do?" He replied: "Humility." Ibn Abbas said: "That is the crouching of a dog. I saw the Prophet, peace and blessings of Allah be upon him, when he prostrated, the whiteness of his armpits could be seen."