Musnad Ahmad 27:1828

Ibn Numayr narrated to us, Ubayd Allah narrated to us, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) caught up with Umar ibn al-Khattab while he was among a group of riders, and he was swearing by his father. The Prophet (peace and blessings of Allah be upon him) said: "Verily, Allah forbids you from swearing by your fathers. So whoever is to swear, let him swear by Allah or let him remain silent."