Sahih Al Bukhari Hadith Number 2001

Narrated `Aisha: Allah’s Messenger (peace be upon him) ordered (the Muslims) to fast on the day of ‘Ashura’, and when fasting in the month of Ramadan was prescribed, it became optional for one to fast on that day (‘Ashura’) or not.

30  Fasting
Hadith number 2001
Volume no 3
Chapter: 69 (Observing Saum (fast) on the day of ‘Ashura)