Musnad Ahmad 850:13

Aswad ibn Amir and Affan narrated to us, they said: Jarir ibn Hazim narrated to us, he said: I heard al-Hasan, from Abd al-Rahman ibn Samurah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said to me: "O Abd al-Rahman, do not ask for authority, 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." Affan and Aswad agreed in their narration, and he said: "Then offer expiation for your oath, then do that which is better." And Abu al-Ashhab said, regarding al-Hasan in this hadith, that he began with the expiation.