Narrated `Aisha: Allah's Apostle ordered (the Muslims) to fast on the day of 'Ashura', and when fasting in the month of Ramadan was prescribed, it became optional for one to fast on that day ('Ashura') or not.
Hadith ReferenceSahih Bukhari / The Book of As-Saum (The Fasting). / Hadith: 2001
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة