Sahih al-Bukhari Book 33, Hadith 1
Allah's Messenger (ﷺ) used to practice I`tikaf in the last ten days of the month of Ramadan.
With the start of the last ten days of Ramadan, the Prophet (ﷺ) used to tighten his waist belt (i.e. work hard) and used to pray all the night, and used to keep his family awake for the prayers.
(the wife of the Prophet) The Prophet (ﷺ) used to practice I`tikaf in the last ten days of Ramadan till he died and then his wives used to practice I`tikaf after him.