Musannaf Ibn Abi Shayba 7:159

Abu al-Ahwas narrated to us, from Abu Ishaq, from Abd al-Rahman ibn Udhaynah, from his father, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever swears an oath, then sees that which is better than it, let him come to that which is better, and let him offer expiation for his oath."