Musnad Ahmad 27:76

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Salim, from his father: That the Prophet, peace and blessings of Allah be upon him, heard 'Umar saying, "By my father." The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, Allah forbids you from swearing by your fathers. So if one of you is to swear, let him swear by Allah, or let him remain silent." 'Umar said: "I have not sworn by them since then, neither as one who initiates it, nor as one who reports it."