Abdul Rahman bin Khalid bin Musafir (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) saying in his sermon while on the pulpit: "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, let him fast it,'" meaning the day of Ashura.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16800