Musnad Ahmad

Sa’d bin Ubaidah (may Allah have mercy on him) says that once I was sitting with Sayyiduna Ibn Umar (may Allah be pleased with them). After a short while, I got up from there and went to sit with Sa’eed bin Musayyib (may Allah have mercy on him). In the meantime, my Kindi companion came; the color of his face had changed and was pale yellow. As soon as he arrived, he said that a man came to Sayyiduna Ibn Umar (may Allah be pleased with them) and said, “O Abu Abdur Rahman! If I swear by the Ka’bah, will there be any sin upon me?” He replied, “No, but if you want to swear by the Ka’bah, then swear by the Lord of the Ka’bah, because Sayyiduna Umar (may Allah be pleased with him) used to swear by his father. One day the Prophet (peace and blessings be upon him) said: ‘Do not swear by your fathers, for whoever swears by other than Allah commits shirk (associating partners with Allah).’”