Imam Malik (may Allah have mercy on him) narrates, through Imam Ibn Shihab al-Zuhri (may Allah have mercy on him), from Humayd bin Abdul Rahman (may Allah have mercy on him) that he heard Sayyiduna Mu'awiyah (may Allah be pleased with him) on the day of Ashura, in the year he performed Hajj, while he was on the pulpit, saying: "O people of Madinah! Where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) say: 'This day is the day of Ashura; fasting it has not been made obligatory upon you, and I am fasting. So whoever wishes may fast, and whoever wishes may break his fast.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16798