Sahih Muslim 1652 a
The Book of Oaths
It is recommended for the one who swears an oath then sees that something else is better than it; To do that which is better and offer expiation for his oath
Abd al-Rahman b. Samura reported that Allah's Messenger (ﷺ) said to me: Abd al-Rahman b. Samura, don't ask for authority for if it is granted to you for asking for it, you would be commissioned for it (without having the support of Allah), but if you are granted it without your asking for it. You would be helped (by Allah) in it. And when you take an oath and find something else better than that, expiate for (breaking) your oath, and do that which is better. This hadith has also been transmitted on the authority of Ibn Farrukh. The above hadith is narratted through another chain of transmitters.