Sahih Bukhari Volume 006, Book 060, Hadith Number 028.
Narrated By Ibn 'Umar : Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-lslamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not."