Sahih Bukhari Volume 008, Book 078, Hadith Number 618.

Narated By '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."