Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 609
Narrated Ibn 'Umar:
When we returned from (the battle of) Hunain, 'Umar asked the Prophet about a vow which he had made during the Pre-lslamic period of Ignorance that he would perform Itikaf. The Prophet ordered him to fulfill his vow.