Sahih Bukhari
Retiring to a Mosque for Remembrance of Allah (Itikaf)
Volume: 003 / Book Number: 033 / Hadith Number: 243
Narrated 'Aisha:
(the wife of the Prophet) The Prophet used to practice Itikaf in the last ten days of Ramadan till he died and then his wives used to practice Itikaf after him.