Musnad Ahmad 1153:600

Qutaybah ibn Sa'id narrated to us, he said: Layth ibn Sa'd narrated to us, from 'Uqayl, from al-Zuhri, from 'Urwah, from 'Aishah: "That the Prophet, peace and blessings of Allah be upon him, used to perform I'tikaf during the last ten days of Ramadan, until Allah took his soul. Then his wives performed I'tikaf after him."