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

Bukhari :: Book 3 :: Volume 33 :: Hadith 258

Narrated 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.