Musnad Ahmad

Abu al-Khasib says that once I was sitting when Sayyiduna Ibn Umar (may Allah be pleased with them both) came. A man stood up, leaving his place, but Sayyiduna Ibn Umar (may Allah be pleased with them both) did not sit there; instead, he went and sat somewhere else. That man said, "If you had sat there, there would have been no harm in it, would there?" He replied, "I cannot sit in your place or anyone else's place because I have seen that once in the gathering of the Prophet (peace and blessings be upon him), a man came, another person stood up and vacated his place for him, and the newcomer was about to sit in his place, but the Prophet (peace and blessings be upon him) forbade him from doing so."