Hadith #589 (Riyad as-Salihin)
Book: The Book of I'tikaf

The Messenger of Allah (ﷺ) used to observe I`tikaf in the last ten days of Ramadan.
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Source: https://parallelquran.com/hadith/riyad-as-salihin-589/

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Hadith #590 (Riyad as-Salihin)
Book: The Book of I'tikaf

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..

Source: https://parallelquran.com/hadith/riyad-as-salihin-590/

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Hadith #591 (Riyad as-Salihin)
Book: The Book of I'tikaf

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..

Source: https://parallelquran.com/hadith/riyad-as-salihin-591/