بُلُوغُ الْمَرَامِ
Bulugh al-Maram 699
Fasting
وَعَنْهَا: { أَنَّ اَلنَّبِيَّ - صلى الله عليه وسلم -كَانَ يَعْتَكِفُ اَلْعَشْرَ اَلْأَوَاخِرَ مِنْ رَمَضَانَ, حَتَّى تَوَفَّاهُ اَللَّهُ, ثُمَّ اعْتَكَفَ أَزْوَاجُهُ مِنْ بَعْدِهِ } مُتَّفَقٌ عَلَيْهِ [1] .
'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.
Arabic/English book reference: Book 5, Hadith 719