It is narrated from Humaid bin Abdur Rahman (may Allah have mercy on him) that he heard Sayyiduna Muawiyah bin Abi Sufyan (may Allah be pleased with him) on the day of Ashura, in the year of Hajj, while Sayyiduna Muawiyah (may Allah be pleased with him) was on the pulpit saying: "O people of Madinah! Where are your scholars? (Why do they not forbid you from attaching artificial hair?) I heard the Messenger of Allah (ﷺ) say: 'Today is the day of Ashura. Fasting on this day has not been made obligatory by Allah, but I am fasting. Whoever wishes may fast, and whoever wishes may break his fast.'"
Hadith Referenceالسنن المأثورة / باب ما جاء في صيام عاشوراء / 329
Hadith Takhrijصحیح بخاری، الصوم، باب صوم يوم عاشوراء، رقم : 2003، صحیح مسلم، الصيام، باب صوم يوم عاشوراء، رقم : 1129