Musnad Ahmad 30:1315

Yahya ibn Adam narrated to us, Abu Bakr ibn Ayyash narrated to us, from Abu Hasin, from Abu Salih, from Abu Hurairah, from the Prophet, peace and blessings be upon him: "He used to perform i'tikaf during the last ten days of the month of Ramadan. When it was the year in which he passed away, he performed i'tikaf for twenty days."