Sahih Bukhari Volume 008, Book 078, Hadith Number 619.

Narated By 'Abdur-Rahman bin Samura : The Prophet said, "O 'Abdur-Rahman bin Samura! Do not seek to be a ruler, because if you are given authority for it, then you will be held responsible for it, but if you are given it without asking for it, then you will be helped in it (by Allah): and whenever you take an oath to do something and later you find that something else is better than the first, then do the better one and make expiation for your oath."