Sahih Muslim Book 15, Hadith Number 4076.
Chapter : What a non-believer should do with his vow when he embraces Islam.
Nafi’ reported: A mention of Allah’s Messenger (may peace be upon him) observing ‘Umra from ja’rina was made before Ibn ‘Umar. He said: He did not enter into the state of Ihram from that (place), and Umar had taken a vow of observing I’tikaf for a night during the days of Ignorance. The rest of the hadith is the same.