Sahih Muslim 2648
صحيح مسلم#2648SahihAbd al-Rahman b. Yazid said:When al-Ash'ath b. Qais entered the house of 'Abdullah he was having his breakfast. He ('Abdullah b. Umar) said: Abd Muhammad (al-Asha'th), come near to the breakfast. Thereupon he said: Is not today the day of 'Ashura? He ('Abd al-Rahman) said: Do you know what the day of 'Ashura is? He said: What is it? He said: It is a day on which the Messenger of Allah (ﷺ) used to observe fast before the (fasting) in the month of Ramadan (became) obligatory. But when it became obligatory the (fasting of 'Ashura) was abandoned (as compulsory). Abu Kuraib said: He (the Holy Prophet) abandoned 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: 13:154