Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) narrates from Sayyiduna Mu'awiyah (may Allah be pleased with him); he said: The Messenger of Allah (peace and blessings be upon him) addressed us on the day of Ashura and said: "This is a day whose fasting has not been made obligatory upon you, and I am fasting. So whoever wishes to fast, let him fast, and whoever wishes not to fast, let him not fast."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16757