Sahih Al Bukhari Hadith Number 2005

Narrated Abu Musa: The day of ‘Ashura’ was considered as `Id day by the Jews. So the Prophet (peace be upon him) ordered, “I recommend you (Muslims) to fast on this day.”

30  Fasting
Hadith number 2005
Volume no 3
Chapter: 69 (Observing Saum (fast) on the day of ‘Ashura.)