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

Bukhari :: Book 3 :: Volume 33 :: Hadith 248

Narrated Ibn 'Umar:

Umar asked the Prophet "I vowed in the Pre-lslamic period of ignorance to stay in Itikaf for one night in Al-Masjid al-Haram." The Prophet said to him, "Fulfill your vow."