Sahih Muslim Book - 14, Hadith - 2784

وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا لَيْثٌ عَنْ عُقَيْلٍ عَنِ الزُّهْرِىِّ عَنْ عُرْوَةَ عَنْ عَائِشَةَ - رضى الله عنها - أَنَّ النَّبِىَّ -صلى الله عليه وسلم- كَانَ يَعْتَكِفُ الْعَشْرَ الأَوَاخِرَ مِنْ رَمَضَانَ حَتَّى تَوَفَّاهُ اللَّهُ عَزَّ وَجَلَّ ثُمَّ اعْتَكَفَ أَزْوَاجُهُ مِنْ بَعْدِهِ.

It was narrated that Aishah [may Allah be pleased with her] that the Prophet (SAW) used to observe I‘tikaf during the last ten days of Ramadan, until Allah took his soul in death, then his wives observed I‘tikaf after him.