Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet Muhammad (peace and blessings be upon him) said regarding fasting on the day of Ashura, this was after the command of fasting in Ramadan was revealed: "Whoever wishes may fast on this day, and whoever wishes may leave it."
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3622
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الصحيحة» -أيضا- (2111): ق.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح