(32) باب مَنْ نَذَرَ فِي الْجَاهِلِيَّةِ ثُمَّ أَدْرَكَ الإِسْلاَمَ
Narrated Ibn 'Umar:
That 'Umar said: Messenger of Allah, I took a vow in pre-Islamic times that I would stay in the sacred mosque (Masjid Haram) as a devotion (i'tikaf). The Prophet (ﷺ) said: Fulfill your vow.
Reference: Sunan Abi Dawud 3325
In-book reference: Book 22, Hadith 84
English translation: Book 21, Hadith 3319