Musnad Ahmad 27:773

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, and 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, but the Prophet, peace and blessings of Allah be upon him, forbade him from it and said: 'Indeed, it is shirk (polytheism).'"