Sahih Bukhari Volume 3, Book 33, Hadith Number 258.

Narated By Abdullah bin Umar : ‘Umar bin Al-Khattab said, “O Allah’s Apostle! I vowed in the Pre-lslamic period to perform Itikaf in Al-Masjid-al-Haram for one night.” The Prophet said, “Fulfill your vow.” So, he performed Itikaf for one night.