From Sahih Bukhari (Hadith 6697):
Narrated Ibn `Umar:`Umar said "O Allah's Messenger (ﷺ)! I vowed to perform I`tikaf for one night in Al-Masjid-al-Haram, during the Pre-Islamic Period of ignorance (before embracing Islam). "The Prophet (ﷺ) said, "Fulfill your vow." Ibn `Umar said to the lady, "Pray on her behalf." Ibn `Abbas said the same
Reference: Bukhari 6697