From Sahih Muslim (Hadith 2653):
Abd al-Rahman reported that he heard Mu'awiya b. Abu Sufyan delivering a sermon in Medina. i. e. when he came there (for Hajj). He delivered a sermon on the day of 'Ashura and said:People of Medina, where are your scholars? I heard the Messenger of Allah (ﷺ) say on this very day: It is the day of 'Ashura. Allah has not made fasting on This day obligatory for you but I am fasting. He who likes to observe fast among you should do so, and he who likes not to observe it may not observe it
Reference: Muslim 2653