Sahih al-Bukhari 4501
صحيح البخاري#4501SahihNarrated Ibn `Umar:Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-Islamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not
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: 65:28