كتاب الصيام
5  
Fasting
Bulugh al-Maram 699
'A’isha (RAA) narrated, ’The Messenger of Allah (ﷺ) used to perform I'tikaf during the last ten days of Ramadan until he died. Then his wives used to perform I'tikaf after his death.’ Agreed upon.
وَعَنْهَا: { أَنَّ اَلنَّبِيَّ ‏- صلى الله عليه وسلم ‏-كَانَ يَعْتَكِفُ اَلْعَشْرَ اَلْأَوَاخِرَ مِنْ رَمَضَانَ, حَتَّى تَوَفَّاهُ اَللَّهُ, ثُمَّ اعْتَكَفَ أَزْوَاجُهُ مِنْ بَعْدِهِ } مُتَّفَقٌ عَلَيْهِ 1‏ .‏
Reference : Bulugh al-Maram 699
In-book reference : Book 5, Hadith 699
English translation : Book 5, Hadith 719
Hadith Text
Reference