Imam Sufyan bin Uyaynah (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), who said: Sayyiduna Mu'awiyah (may Allah be pleased with him) came to Madinah and said: "Where are your scholars? The Messenger of Allah (peace and blessings be upon him) was on this day and he (peace and blessings be upon him) said: 'I am fasting, so whoever wishes may fast and whoever wishes may break his fast,'" meaning the day of Ashura.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16799