Musannaf Ibn Abi Shayba 35:67

Hafs narrated to us, from Ubayd Allah ibn Umar, from Nafi, from Ibn Umar, from Umar, who said: I made a vow during the Jahiliyyah (pre-Islamic period), so I asked the Prophet, peace and blessings of Allah be upon him, after I had accepted Islam. He commanded me to fulfill my vow.