Sahih al-Bukhari 3397
صحيح البخاري#3397SahihNarrated Ibn `Abbas:When the Prophet (ﷺ) came to Medina, he found (the Jews) fasting on the day of 'Ashura' (i.e. 10th of Muharram). They used to say: "This is a great day on which Allah saved Moses and drowned the folk of Pharaoh. Moses observed the fast on this day, as a sign of gratitude to Allah." The Prophet (ﷺ) said, "I am closer to Moses than they." So, he observed the fast (on that day) and ordered the Muslims to fast on 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 al-Bukhari is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 60:70