Musannaf Ibn Abi Shayba 7:136

Waki’ narrated to us, from al-A’mash, from Sa’d ibn ‘Ubaydah, who said: We were with ‘Umar in a circle, and he heard a man saying, "No, and by my father." He threw pebbles at him and said: "It was my oath, and the Prophet, peace and blessings of Allah be upon him, forbade it and said: 'It is shirk (polytheism).'"