Narrated `Aisha:
The people used to fast on the day of 'Ashura' before fasting in Ramadan was prescribed but when (the order of compulsory fasting in) Ramadan was revealed, it was up to one to fast on it (i.e. 'Ashura') or not.
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 29
Arabic reference: Book 65, Hadith 4502