Sahih Muslim | Book 6 | Hadith Number 2515
Narrated by Abd al-Rahman
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 (may peace be upon him) 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.