Imam al-Zuhri (may Allah have mercy on him) said: Humayd bin Abdul Rahman (may Allah have mercy on him) told me 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, and fasting it has not been made obligatory upon us. So whoever among you wishes to fast, let him fast, for I am fasting.'" So the people fasted.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16797