Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)

Bukhari :: Book 5 :: Volume 59 :: Hadith 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.