Musnad Ahmad 850:12

Abdullah ibn Bakr narrated to us, Hisham narrated to us, from al-Hasan, from Abd al-Rahman ibn Samurah, from the Prophet, may Allah bless him and grant him peace, that he said to him: "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 offer expiation for your oath, and do that which is better."