Ibn 'Umar (Allah be pleased with both of them) reported that the Messenger of Allah ﷺ used to observe i'tikaf in the last ten days of Ramadan.

أَنَّ النَّبِيَّ ﷺ كَانَ يَعْتَكِفُ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ