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. (Using translation from Abū Dāʾūd 2466)   

كَانَ رَسُولُ اللهِ ﷺ يَعْتَكِفُ فِي كُلِّ رَمَضَانَ عَشَرَةَ أَيَّامٍ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ يَوْمًا  

The Prophet ﷺ used to spend the last ten days of Ramadan in I’tikaf. One year he was traveling, so the following year he spent twenty days in I’tikaf. (Using translation from Ibn Mājah 1770)   

أَنَّ رَسُولَ اللهِ ﷺ كَانَ يَعْتَكِفُ الْعَشْرَ الْأَوَاخِرَ مِنْ رَمَضَانَ فَسَافَرَ عَامًا فَلَمْ يَعْتَكِفْ فَلَمَّا كَانَ مِنْ قَابِلٍ اعْتَكَفَ عِشْرِينَ يَوْمًا وَرُوِيَ فِي ذَلِكَ عَنْ أَنَسِ بْنِ مَالِكٍ  

[Machine] The Messenger of Allah, ﷺ , used to observe voluntary seclusion during the last ten days of Ramadan when he was residing, and during the next year he observed voluntary seclusion for twenty days when he was traveling.  

كَانَ رَسُولُ اللهِ ﷺ إِذَا كَانَ مُقِيمًا اعْتَكَفَ الْعَشْرَ الْأَوَاخِرَ وَإِذَا سَافَرَ اعْتَكَفَ الْعَامَ الْمُقْبِلَ عِشْرِينَ يَوْمًا