Retiring to a Mosque for Remembrance of Allah (I'tikaf)

Bukhari :: Book 3 :: Volume 33 :: Hadith 259

Narrated 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."