"The Prophet would perform I'tikaf during the last ten (nights) of Ramadan. One year he did not perform I'tikaf, so he performed I'tikaf for twenty (nights) in the following year."  

كَانَ النَّبِيُّ ﷺ يَعْتَكِفُ فِي الْعَشْرِ الأَوَاخِرِ مِنْ رَمَضَانَ فَلَمْ يَعْتَكِفْ عَامًا فَلَمَّا كَانَ فِي الْعَامِ الْمُقْبِلِ اعْتَكَفَ عِشْرِينَ