Musnad Abu Ya'la al-Mawsili 107:11

Abu Yahya Kamil ibn Talhah narrated to us, Ibn Lahi'ah narrated to us, Ka'b ibn Alqamah narrated to us, from Abd al-Rahman ibn Shamasa al-Mahri, 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 is an oath, and its expiation is the expiation of an oath."