Musnad Ahmad 28:497

Surayj ibn al-Nu‘man narrated to us, Ibn Abi al-Zinad narrated to us, from ‘Abd al-Rahman ibn al-Harith, from ‘Amr ibn Shu‘ayb, from his father, from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) looked at a Bedouin standing in the sun while he (the Prophet) was delivering a sermon. He said: "What is your matter?" He said: "I vowed, O Messenger of Allah, that I would not cease to be in the sun until you finish." The Messenger of Allah (may Allah bless him and grant him peace) said: "This is not a vow. The vow is only that by which the Face of Allah, the Almighty and Majestic, is sought."