Indeed, Allah Almighty forbids you to swear by your forefathers

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "Indeed, Allah Almighty forbids you to swear by your forefathers." ‘Umar said: By Allah, I have never sworn by that since I heard that the Messenger of Allah (may Allah's peace and blessings be upon him) forbade it, neither deliberately nor reporting it from others.

The Prophet (may Allah's peace and blessings be upon him) informed us that Allah Almighty forbids swearing by the forefathers. So, whoever wants to swear should swear by Allah only and none but Him. ‘Umar ibn al-Khattāb, then, mentioned that ever since he heard the Prophet (may Allah's peace and blessings be upon him) prohibit it, he never swore by other than Allah neither intentionally nor by reporting such an oath on behalf of others.