Sahih al-Bukhari 4504
صحيح البخاري#4504SahihNarrated 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
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 65:31