Sahih Muslim Book 15, Hadith Number 4054.

Chapter : It is excellent to break the vow if one finds it better doing that which is against this vow one should expiate it.

Abu Huraira reported Allah’s Messenger (may peace be upon him) as saying: He who took an oath and (later on) found another thing better than that, he should do that which is better, and expiate for the vow (broken by him).