Sahih al-Bukhari 3831
صحيح البخاري#3831Sahihحَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، قَالَ هِشَامٌ حَدَّثَنِي أَبِي، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كَانَ عَاشُورَاءُ يَوْمًا تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ، وَكَانَ النَّبِيُّ صلى الله عليه وسلم يَصُومُهُ، فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ، فَلَمَّا نَزَلَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَهُ، وَمَنْ شَاءَ لاَ يَصُومُهُ.
Narrated `Aisha:'Ashura' (i.e. the tenth of Muharram) was a day on which the tribe of Quraish used to fast in the pre-Islamic period of ignorance. The Prophet (ﷺ) also used to fast on this day. So when he migrated to Medina, he fasted on it and ordered (the Muslims) to fast on it. When the fasting of Ramadan was enjoined, it became optional for the people to fast or not to fast on the day of Ashura
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:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 63:56