(36) باب إِذَا نَذَرَ ثُمَّ أَسْلَمَ قَبْلَ أَنْ يَفِيَ
It was narrated from Ibn 'Umar that 'Umar had vowed -during the Jahiliyyah- to spend a day in 'Itikaf. He asked the Messenger of Allah about that, and he commanded him to perform the 'Itikaf. (ID)
Reference (ID): Sunan an-Nasa'i 3822 (ID)
In-book reference (ID): Book 35, Hadith 62 (ID)
English translation (ID): Vol. 4, Book 35, Hadith 3853 (ID)