From Sahih Bukhari (Hadith 6621):
Narrated `Aisha:Abu Bakr As-Siddiq had never broken his oaths till Allah revealed the expiation for the oaths. Then he said, "If I take an oath to do something and later on I find something else better than the first one, then I do what is better and make expiation for my oath
Reference: Bukhari 6621