Sahih Bukhari Volume 3, Book 33, Hadith Number 259.

Narated By Ibn ‘Umar : That ‘Umar had vowed in the Pre-lslamic period to perform Itikaf in Al-Masjid-al-Haram. (A sub-narrator thinks that ‘Umar vowed to perform Itikaf for one night.) Allah’s Apostle said to ‘Umar, “Fulfill your vow.”