Riyad as-Salihin
Ibn `Umar (May Allah be pleased with them) reported: The Messenger of Allah (ﷺ) used to observe I`tikaf in the last ten days of Ramadan.
[Al-Bukhari and Muslim].
Reference: Riyad as-Salihin 1268
In-book reference: Book 9, Hadith 1
'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].
Reference: Riyad as-Salihin 1269
In-book reference: Book 9, Hadith 2
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].
Reference: Riyad as-Salihin 1270
In-book reference: Book 9, Hadith 3