Sahih Bukhari
Expiation for Unfulfilled Oaths
Volume: 008 / Book Number: 079 / Hadith Number: 715
Narrated 'Abdur-Rahman bin Samura:
Allah's Apostle said, "(O 'Abdur-Rahman!) Do not seek to be a ruler, for, if you are given the authority of ruling without your asking for it, then Allah will help you; but if you are given it by your asking, then you will be held responsible for it (i.e. Allah will not help you ) . And if you take an oath to do something and later on find another thing, better than that, then do what is better and make expiation for (the dissolution of) your oath."