It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said: "Whoever wants to swear an oath, let him swear by the name of Allah." The people of Quraysh used to swear oaths by the names of their forefathers, so he said: "Do not swear oaths by the names of your forefathers."