Sahih Muslim Book 6, Hadith Number 2640.

Chapter : Observing of I’tikaf in the last ten days of Ramadhan.

‘Aisha (Allah be pleased with her) reported that the Messenger of Allah (may peace he upon him) used to observe i’tikaf in the last ten days of Ramadan till Allah called him back (to his heavenly home). Then his wives observed i’tikaf after him.