Sahih Muslim 1650 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
Abu Huraira reported: A person sat late in the night with Allah's Messenger (ﷺ), and then came to his family and found that his children had gone to sleep. His wife brought food for him. but he took an oath that he would not eat because of his children (having gone to sleep without food) He then gave precedence (of breaking the vow and then expiating it) and ate the food He then came to Allah s Messenger (ﷺ) and made mention of that to him, whereupon Allah's Messenger (ﷺ) said: He who took an oath and (later on) found something better than that should do that, and expiate for (breaking) his vow.