Umm al-Mu'mineen Sayyidah Aisha (may Allah be pleased with her) narrates: The Quraish used to fast on the day of Ashura even in the days of ignorance, and the Messenger of Allah (peace and blessings be upon him) also used to fast on that day. When the Messenger of Allah (peace and blessings be upon him) came to Madinah, he fasted on the day of Ashura and also ordered others to fast on it. Then, when the fasts of Ramadan were made obligatory, the obligation of fasting on Ashura was abrogated; whoever wished could fast, and whoever wished could leave it.
Hadith Referenceالسنن المأثورة / باب ما جاء في صيام عاشوراء / 328
Hadith Takhrijصحیح بخاری، الصوم، باب صوم يوم عاشوراء، رقم : 2022، صحیح مسلم، الصيام، باب صوم يوم عاشوراء، رقم : 1125