Bismillah Walhamdulillah Was Salaatu Was Salaam ‘ala Rasulillah

Narrated ‘Aisha (Radi-Allahu ‘anha):

Allah’s Apostle (Sallallahu ‘Alaihi Wa Sallam) 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.

Bukhari Vol. 3 : No. 219