Musnad Ahmad 482:132

Ishaq ibn Isa narrated to us, Ibn Lahi'ah narrated to us, Ka'b ibn Alqamah narrated to us, from Abd al-Rahman ibn Shamasa, from Abu al-Khayr, from Uqbah ibn Amir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The vow, its expiation is only the expiation of an oath."