Musnad Ahmad 27:220

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' informed me, from Ibn 'Umar, that the Messenger of Allah (ﷺ) caught up with 'Umar while he was among a group of riders, and he was swearing by his father. He said: "Do not swear by your fathers. Whoever is to swear, let him swear by Allah or let him remain silent."