Narrated 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."
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 28
Arabic reference: Book 65, Hadith 4501