Sahih Bukhari | Volume 3 | Hadith Number 117

Narrated by 'Aisha
(The tribe of) Quraish used to fast on the day of Ashura' in the Pre-Islamic 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."