Musnad Ahmad 27:807

Waki’ narrated to us, al-A’mash narrated to us, from Sa’d ibn ‘Ubaydah, who said: I was with Ibn ‘Umar in a circle. He said: He heard a man in another circle saying, "No, and by my father." Ibn ‘Umar threw pebbles at him and said: "It was an oath of ‘Umar, and the Prophet, may Allah bless him and grant him peace, forbade him from it and said: 'Indeed, it is shirk (polytheism).'"