Riyad us Saliheen

The Messenger of Allah (ﷺ) used to observe I`tikaf in the last ten days of Ramadan.

[Al-Bukhari and Muslim].

The Prophet (ﷺ) used to engage himself in I'tikaf (seclusion for prayers) in the mosque during the last ten nights of Ramadan till he passed away; thereafter, his wives followed this practice after him.

[Al- Bukhari and Muslim].

The Prophet (ﷺ) used to observe I'tikaf every year (during Ramadan) for ten days; in the year in which he passed away, he observed I'tikaf for twenty days.

[Al-Bukhari].