Sahih Bukhari Volume 5, Book 58, Hadith Number 172.

Narated By ‘Aisha : ‘Ashura’ (i.e. the tenth of Muharram) was a day on which the tribe of Quraish used to fast in the pre-lslamic period of ignorance. The Prophet also used to fast on this day. So when he migrated to Medina, he fasted on it and ordered (the Muslims) to fast on it. When the fasting of Ramadan was enjoined, it became optional for the people to fast or not to fast on the day of Ashura.