Musnad Ahmad 435:40

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, who said: Humayd ibn 'Abd al-Rahman ibn 'Awf narrated to me that he heard Mu'awiyah delivering a sermon in Medina, saying: "O people of Medina, where are your scholars? I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: 'This is the day of Ashura, and its fasting has not been made obligatory upon us. So whoever among you wishes to fast, let him fast, for I am fasting.'" So they fasted.