Sahih Muslim 2666
صحيح مسلم#2666SahihIbn 'Abbas reported that when the Messenger of Allah (ﷺ) fasted on the day of 'Ashura and commanded that it should he observed as a fast, they (his Companions) said to him:Messenger of Allah, it is a day which the Jews and Christians hold in high esteem. Thereupon the Messenger of Allah (ﷺ) said: When the next year comes, God willing, we would observe fast on the 9th But the Messenger of Allah (ﷺ) died before the advent of the next year
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: 13:172