Musnad Abu Ya'la al-Mawsili 2:115

Narrated to us Abdullah ibn Umar ibn Aban, narrated to us Hafs ibn Ghiyath, from Ubaydullah, from Nafi, from Ibn Umar, from Umar, who said: I said, "O Messenger of Allah, I made a vow during the Jahiliyyah (pre-Islamic period), then Allah brought Islam." He said, "Fulfill your vow."