Musnad Ahmad 2:247

Abu Sa'id narrated to us, Isra'il narrated to us, Sa'id ibn Masruq narrated to us, from Sa'd ibn 'Ubaydah, from Ibn 'Umar, from 'Umar, that he said: "No, and by my father." The Messenger of Allah (peace and blessings of Allah be upon him) said: "Desist. Indeed, whoever swears by something other than Allah has committed shirk (polytheism)."