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