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.
Hadith ReferenceSahih Bukhari / The Book of Commentary / Hadith: 4502
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة