The Prophet ﷺ used to observe I'tikaf during the last ten days of Ramadan. Nafi' said: 'Abd Allah (b. 'Umar) showed me the place in the mosque where Messenger of Allah ﷺ used to observe I'tikaf.  

أَنَّ النَّبِيَّ ﷺ كَانَ يَعْتَكِفُ الْعَشْرَ الأَوَاخِرَ مِنْ رَمَضَانَ قَالَ نَافِعٌ وَقَدْ أَرَانِي عَبْدُ اللَّهِ الْمَكَانَ الَّذِي يَعْتَكِفُ فِيهِ رَسُولُ اللَّهِ ﷺ مِنَ الْمَسْجِدِ  

The Prophet ﷺ used to observe I'tikaf during ten days of Ramadan every year. But when the year in which he died, he observed I'tikaf for twenty days.  

كَانَ النَّبِيُّ ﷺ يَعْتَكِفُ كُلَّ رَمَضَانَ عَشَرَةَ أَيَّامٍ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ يَوْمًا