Musnad Abu Ya'la al-Mawsili 110:62

Zuhayr narrated to us, Ibn 'Uyaynah narrated to us, from 'Amr, from Tawus, from Ibn 'Abbas: The Messenger of Allah (may Allah bless him and grant him peace) commanded that one should prostrate upon seven [bones], and he forbade that one should tuck up his hair and his garments.