Sahih Bukhari
Sahih Bukhari Volume 003, Book 031, Hadith Number 219.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 31. Fasting
Narated By 'Aisha : Allah's Apostle 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.