Humaid bin Abdur Rahman narrates: Once, Sayyiduna Muawiyah (may Allah be pleased with him) came to Madinah Munawwarah. He delivered a sermon on the day of Ashura and said: O people of Madinah! Where are your scholars? I heard the Prophet Muhammad (peace and blessings be upon him) say this: "This is the day of Ashura. Fasting on this day has not been made obligatory upon you, but I am fasting. So whoever wishes to fast on this day may do so."
Hadith Referenceصحیح ابن حبان / كتاب الصوم / 3626
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح: ق.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم