Musnad Ahmad 482:35

'Attab—meaning Ibn Ziyad—narrated to us, Abdullah—meaning Ibn al-Mubarak—narrated to us, Yahya ibn Ayyub informed us, Ka'b ibn 'Alqamah narrated to me that he heard Abd al-Rahman ibn Shamasa narrating from Abu al-Khayr, who said: I heard Uqbah ibn Amir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "The expiation for a vow is the expiation for an oath."