Abdul Rahman bin Ishaq (may Allah have mercy on him) narrates, through Imam al-Zuhri (may Allah have mercy on him), from Humayd bin Abdul Rahman bin Awf (may Allah have mercy on him) that he heard Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) delivering a sermon on the day of Ashura. He said: "O people of Madinah! Where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) say: 'This is the day of Ashura; fasting it has not been made obligatory upon you, and I am fasting. So whoever wishes to fast it, let him fast it, and whoever wishes to break his fast, let him do so.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16801