Musnad Abu Ya'la al-Mawsili 111:57

Amr ibn Muhammad al-Naqid narrated to us, Waki’ ibn al-Jarrah narrated to us, from a sheikh whom Waki’ named, who said: I heard Tawus narrating from Abdullah al-Azdi or Ubaydullah al-Azdi—Abu Uthman doubted—from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It has been revealed to me that I should prostrate upon seven bones, and that I should not tuck up hair nor clothing."