Sayyiduna Ibn Umar (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Whoever wishes to swear, let him swear only by Allah." The Quraysh used to swear by their forefathers, so he (peace and blessings be upon him) said: "Do not swear by your forefathers."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 19832