Narrated ‘Aisha:
(The tribe of) Quraish used to fast on the day of Ashura’ in the Pre-lslamic period, and then Allah’s Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet said, “He who wants to fast (on ‘Ashura’) may fast, and he who does not want to fast may not fast.”
Sahih Bukhari, Volume 3, Book31, Number 117