Sunan Abi Dawud 2466
سنن أبي داود#2466Sahihحَدَّثَنَا هَنَّادٌ، عَنْ أَبِي بَكْرٍ، عَنْ أَبِي حَصِينٍ، عَنْ أَبِي صَالِحٍ، عَنْ أَبِي هُرَيْرَةَ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم يَعْتَكِفُ كُلَّ رَمَضَانَ عَشَرَةَ أَيَّامٍ فَلَمَّا كَانَ الْعَامُ الَّذِي قُبِضَ فِيهِ اعْتَكَفَ عِشْرِينَ يَوْمًا .
Abu Hurairah said: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
Scholarly Authentication
The label above is the grade most of these scholars gave — one dissenting opinion no longer speaks for all of them. If any scholar calls a narration fabricated, that verdict is shown instead, whatever the others say. Each scholar's own wording:
Al-AlbaniHasan Sahih
Muhammad Muhyi Al-Din Abdul HamidHasan Sahih
Shuaib Al ArnautSahih
Zubair Ali ZaiSahih Bukhari (2044)
In-book reference: 14:154