Yahya related to me from Malik from Ibn Shihab that Humayd ibnAbd ar-Rahman ibn Awf heard Muawiya ibn Abi Sufyan say from the mimbaron the day of Ashura in the year in which he made the hajj, "People ofMadina, where are your learned men? I heard the Messenger of Allah,may Allah bless him and grant him peace, say about this day, 'This isthe day of Ashura, and fasting it has not been prescribed for you. Iam fasting it, and whoever of you wants to fast it can do so, andwhoever does not want to, does not have to.' "
USC-MSA web (English) reference: Book 18, Hadith 34
Arabic reference: Book 18, Hadith 668