Musnad Ahmad 30:2089

Sulayman ibn Dawud al-Hashimi narrated to us, he said: Abu Bakr ibn Ayyash informed us, from Abu Husayn, from Abu Salih, from Abu Hurayrah, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to perform I'tikaf during the last ten days of Ramadan, and the middle ten days. Then, when he passed away, he passed away while he was performing I'tikaf for twenty days."