Lady Aisha Siddiqa (may Allah be pleased with her) narrates: The day of Ashura was a day on which the Quraysh used to fast during the period of ignorance. When the Prophet Muhammad (peace and blessings be upon him) came to Madinah, he fasted on this day and ordered fasting on this day. When Ramadan was made obligatory, then that (fast) became obligatory and the day of Ashura was abandoned. So whoever wished would fast on (the day of Ashura), and whoever wished would leave it.
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3621
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «صحيح أبي داود» (2110): ق.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط الشيخين