Chapter: The vow of a disbeliever, and what he should do about it if he becomes a muslim
lbn 'Umar reported: When Allah's Apostle ( صلی اللہ علیہ وسلم ) came back from the Battle of Hunain, Umar asked Allah's Messenger ( صلی اللہ علیہ وسلم ) about the vow he had taken during the days of Ignorance that he would observe I'tikaf for a day. The rest of the hadith is the same.
Authentic