Muhammad bin Abi Hafsah (may Allah have mercy on him) narrates, through Imam 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) say on the pulpit: "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 may fast, and whoever wishes may break his fast.'"
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16803