Sahih Muslim 2660
صحيح مسلم#2660SahihAbu Musa (Allah be pleased with him) reported:The day of 'Ashura was one which the Jews respected and they treated it as Id. The Messenger of Allah (ﷺ) said: You also observe fast on this day
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: 13:166