Musannaf Ibn Abi Shayba 7:283

Hafs narrated to us, from Ubayd Allah ibn Umar, from Nafi, from Ibn Umar, from Umar, may Allah be pleased with him, who said: I made a vow during the Jahiliyyah, then I accepted Islam. I asked the Prophet, may Allah bless him and grant him peace, and he commanded me to fulfill my vow.