Sahih Muslim 2637
صحيح مسلم#2637SahihA'isha (Allah be pleased with her) reported that the Quraish used to fast on the day of 'Ashura in the pre-Islamic days and the Messenger of Allah (ﷺ) also observed it. When he migrated to Medina, he himself observed this fast and commanded (others) to observe it. But when fasting during the month of Ramadan was made obligatory he said:He who wishes to observe this fast may do so, and he who wishes to abandon it may do so
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 13:143