NASAI-KUBRA 3329:
from Abū Hurayrah —
[Machine] The passage translates to: "It is narrated from the Prophet ﷺ that he used to observe itikaf during the last ten days of Ramadan. However, in the year he passed away, he observed itikaf for twenty days."
الكبرى للنسائي ٣٣٢٩:
عن ابو هريره —
عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ «يَعْتَكِفُ الْعَشْرَ الْأَوَاخِرَ مِنْ شَهْرِ رَمَضَانَ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ»