Sahih Bukhari
Retiring to a Mosque for Remembrance of Allah (Itikaf)
Volume: 003 / Book Number: 033 / Hadith Number: 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."