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