Messenger of Allah ﷺ used to practice Iʿtikaf in the last ten days of the month of Ramadan.  

كَانَ رَسُولُ اللَّهِ ﷺ يَعْتَكِفُ الْعَشْرَ الأَوَاخِرَ مِنْ رَمَضَانَ  

(the wife of the Prophet) The Prophet ﷺ used to practice Iʿtikaf in the last ten days of Ramadan till he died and then his wives used to practice Iʿtikaf after him.  

زَوْجِ النَّبِيِّ ﷺ أَنَّ النَّبِيَّ ﷺ كَانَ يَعْتَكِفُ الْعَشْرَ الأَوَاخِرَ مِنْ رَمَضَانَ حَتَّى تَوَفَّاهُ اللَّهُ ثُمَّ اعْتَكَفَ أَزْوَاجُهُ مِنْ بَعْدِهِ  

The Prophet ﷺ used to perform Iʿtikaf every year in the month of Ramadan for ten days, and when it was the year of his death, he stayed in Iʿtikaf for twenty days.  

كَانَ النَّبِيُّ ﷺ يَعْتَكِفُ فِي كُلِّ رَمَضَانَ عَشْرَةَ أَيَّامٍ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ يَوْمًا