Musnad Ahmad 850:1

Hushaym narrated to us, Mansur and Yunus informed us, from al-Hasan, from Abd al-Rahman ibn Samurah, who said: The Prophet, peace and blessings of Allah be upon him, said to me: "O Abd al-Rahman ibn Samurah, if you swear an oath, and then you see something else that is better than it, then do that which is better, and offer expiation for your oath."