Sahih al-Bukhari 4504
صحيح البخاري#4504Sahihحَدَّثَنِي مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا يَحْيَى، حَدَّثَنَا هِشَامٌ، قَالَ أَخْبَرَنِي أَبِي، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كَانَ يَوْمُ عَاشُورَاءَ تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ، وَكَانَ النَّبِيُّ صلى الله عليه وسلم يَصُومُهُ، فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ، فَلَمَّا نَزَلَ رَمَضَانُ كَانَ رَمَضَانُ الْفَرِيضَةَ، وَتُرِكَ عَاشُورَاءُ، فَكَانَ مَنْ شَاءَ صَامَهُ، وَمَنْ شَاءَ لَمْ يَصُمْهُ.
Narrated Aisha:During the Pre-Islamic Period of ignorance the Quraish used to observe fasting on the day of 'Ashura', and the Prophet (ﷺ) himself used to observe fasting on it too. But when he came to Medina, he fasted on that day and ordered the Muslims to fast on it. When (the order of compulsory fasting in ) Ramadan was revealed, fasting in Ramadan became an obligation, and fasting on 'Ashura' was given up, and who ever wished to fast (on it) did so, and whoever did not wish to fast on it, did not fast
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: 65:31