Musnad Abu Ya'la al-Mawsili 110:137

Ibrahim ibn Hajjaj narrated to us, Wahib narrated to us, from Ibn Tawus, from his father, from Ibn Abbas: That the Prophet, peace and blessings of Allah be upon him, said: "I have been commanded to prostrate upon seven bones: the forehead"—and he pointed with his hand to his nose—"and the two hands, and the two knees, and the two feet; and I shall not tuck up the garments nor the hair."