(36) باب إِذَا نَذَرَ ثُمَّ أَسْلَمَ قَبْلَ أَنْ يَفِيَ
It was narrated that Ibn 'Umar said:
"Umar had made a vow to spend a night in 'Itikaf in Al-Masjid Al-Haram. He asked the Messenger of Allah about that, and he ordered him to perform the 'Itikaf."
Reference: Sunan an-Nasa'i 3821
In-book reference: Book 35, Hadith 61
English translation: Vol. 4, Book 35, Hadith 3852