Salih bin Kaysan (may Allah have mercy on him) narrates, through Imam Ibn Shihab al-Zuhri (may Allah have mercy on him), that Humayd bin Abdul Rahman (may Allah have mercy on him) told him that he heard Sayyiduna Mu'awiyah (may Allah be pleased with him) delivering a sermon to the people on the day of Ashura, saying: "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,' and he was also saying: 'So whoever among you wishes to fast it, let him fast, and whoever wishes to break his fast, let him do so.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16802