Musnad Ahmad 1153:1337

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, he said: And Ibn Shihab narrated to me, regarding the one who performs i'tikaf and how his sunnah is, from Sa'id ibn al-Musayyib and 'Urwah ibn al-Zubayr, from 'Aishah, the wife of the Prophet, may Allah bless him and grant him peace, that she informed them: "That the Prophet, may Allah bless him and grant him peace, used to perform i'tikaf during the last ten days of the month of Ramadan, until Allah, the Almighty and Majestic, took his soul."