Musnad Ahmad 482:11

Abu Bakr ibn Ayyash narrated to us, he said: Muhammad, the freed slave of al-Mughirah ibn Shu'bah, narrated to me, he said: Ka'b ibn 'Alqamah narrated to me, from Abu al-Khayr Marthad ibn 'Abdullah, from 'Uqbah ibn 'Amir, he said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The expiation for a vow is the expiation for an oath."