Musnad Ahmad 27:456

'Abd al-Razzaq narrated to us, Sufyan informed us, from his father, and al-A'mash, and Mansur, from Sa'd ibn 'Ubaydah, from Ibn 'Umar, who said: 'Umar used to swear: "By my father." The Prophet, peace and blessings of Allah be upon him, forbade him and said: "Whoever swears by something other than Allah the Exalted has committed shirk (polytheism)." And the other [narrator] said: "It is shirk."