Musnad Ahmad 850:7

Hashim ibn al-Qasim narrated to us, al-Mubarak narrated to us, al-Hasan narrated to us, Abd al-Rahman ibn Samurah al-Qurashi narrated to me, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said to me: "O Abd al-Rahman, do not ask for leadership, for if you are given it upon asking, you will be entrusted to it, and if you are given it without asking, you will be aided in it. And if you swear an oath, and then see something else that is better than it, then come to that which is better, and offer expiation for your oath."