Sayyiduna Abdullah bin Amr (may Allah be pleased with him) narrated: On an intensely hot day the Prophet (peace and blessings be upon him) delivered a sermon. He noticed a man standing; he appeared to be a Bedouin and was standing in the sun. The Prophet (peace and blessings be upon him) said to him: "Why is it that I see you standing?" He said: "I have vowed that I will not sit until you finish your sermon." The Prophet (peace and blessings be upon him) said to him: "Sit down! This is not a vow. A vow is that by which the pleasure of Allah Almighty is intended."
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Abdullah bin Nafi' al-Madani, who is da'if (weak).