Musnad Ahmad 30:666

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from 'Urwah, from 'A'ishah, and from Ibn al-Musayyib, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) used to perform i'tikaf during the last ten days of Ramadan, until Allah the Almighty and Majestic took his soul.