Musnad Ahmad 482:49

Abu Sa'id, the freed slave of Banu Hashim, narrated to us, saying: Ibn Lahi'ah narrated to us, saying: Ka'b ibn 'Alqamah narrated to us, saying: I heard 'Abd al-Rahman ibn Shumasah saying: We came to Abu al-Khayr, and he said: I heard 'Uqbah ibn 'Amir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The vow is only an oath; its expiation is the expiation of an oath."