Musnad Ahmad 25:918

Yahya ibn Ishaq narrated to us, he said: Wuhayb ibn Khalid informed us, Abdullah ibn Tawus narrated to us, from his father, from Ibn Abbas, that the Messenger of Allah (may Allah bless him and grant him peace) said: "I have been commanded to prostrate upon seven bones: the forehead—and he pointed with his hand to his nose—, the two hands, the two knees, and the tips of the toes. And I shall not tuck up the garments nor the hair."