Musannaf Ibn Abi Shayba 7:134

Abu al-Ahwas narrated to us, from Simak, from Ikrimah, who said: Umar said: I narrated a hadith to some people, and I said, "No, by my father." A man from behind me said, "Do not swear by your fathers." I turned around, and it was the Messenger of Allah (peace and blessings of Allah be upon him). He said, "If one of you were to swear by the Messiah, he would perish, and the Messiah is better than your fathers."