Sahih Bukhari | Volume 8 | Hadith Number 688
Narrated by Ibn 'Umar
'Umar said "O Allah's Apostle! I vowed to perform Itikaf for one night in Al-Masjid-al- Haram, during the Pre-Islamic Period of ignorance (before embracing Islam). "The Prophet said, "Fulfil your vow." Ibn 'Umar said to the lady, "Pray on her behalf." Ibn 'Abbas said the same.