Sahih Muslim 2644
صحيح مسلم#2644SahihIbn 'Umar (Allah be pleased with them) said that the day of 'Ashura was mentioned before the Messenger of Allah (may peace he upon him). Thereupon the Messenger of Allah, (ﷺ) said:That was a day on which the people of pre-Islamic days used to observe fast. So he who amongst you likes to observe fast should do so, and he who does not like it should abandon it
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: 13:150