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." (Book 79, Hadith 715)
Narrated Abu Musa: Two men from my tribe and I entered upon the Prophet. One of the two men said to the Prophet, "O Allah's Apostle! Appoint me as a governor," and so did the second. The Prophet said, "We do not assign the authority of ruling to those who ask for it, nor to those who are keen to have it." (Book 89, Hadith 263)
(baraka Allaho fee brother Mabsoot for making the hadiths readily available for quotation)