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.
Reference: Sunan an-Nasa'i 3822 In-book : Book 35, Hadith 62 English translation : Vol. 4, Book 35, Hadith 3853
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