Imam al-Zuhri (may Allah have mercy on him) narrates from Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) that he heard Sayyiduna Mu'awiyah (may Allah be pleased with him) delivering a sermon, 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. Allah has not made its fasting obligatory, and I am fasting. So whoever wishes to fast, let him fast, and whoever wishes not to fast, let him not fast."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16765