Sahih Muslim Book 15, Hadith Number 4053.

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 then found another thing better than (this) should expiate for the oath (broken) by him and do (the better thing).