Musnad Ahmad 27:258

Yahya narrated to us, from Ubaydullah, Nafi’ narrated to me, from Abdullah ibn Umar. Abdullah ibn Ahmad said: My father said: And Yahya ibn Sa’id said, one time, from Umar that he said: O Messenger of Allah, I vowed during the Jahiliyyah to seclude myself (i’tikaf) for a night in the Masjid. He said: "Fulfill your vow."