Musnad Ahmad 850:10

Muhammad ibn Abi ‘Adi narrated to us, from Ibn ‘Awn, from al-Hasan, from ‘Abd al-Rahman ibn Samurah, who said: The Prophet, peace and blessings of Allah be upon him, mentioned [the matter of leadership] and said: "Do not ask for leadership, for if you are given it without asking, you will be helped with it; but if you are given it because of asking, you will be left to it. And if you swear an oath and see that something else is better than it, then do that which is better, and offer expiation for your oath."