Riyad as-Salihin
The Book of I'tikaf
كتاب الاعتكاف
Chapter 1: I'tikaf (Seclusion in the Mosque) in the Month of Ramadan
'Aishah (May Allah be pleased with her) reported
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].
Abu Hurairah (May Allah be pleased with him) reported
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].