Humayd bin Abdur-Rahman bin Awf (may Allah have mercy on him) narrates that he heard Sayyiduna Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) on the day of Ashura, in the year he performed Hajj, saying while he was on the pulpit: O people of Madinah, where are your scholars? I heard the Messenger of Allah (peace and blessings be upon him) saying: "This day is the day of Ashura. Allah Almighty has not made its fasting obligatory upon you; whoever wishes may fast on it, and whoever wishes may not."
(Al-Shafi'i added in his narration: "And I am fasting, so whoever wishes, let him fast," and the rest of the narration has the same meaning. His saying "Allah has not made its fasting obligatory upon you" indicates that it was never obligatory, because 'lam' refers to the past.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8416