Maliks Muwatta Book 18, Hadith Number 34.

Section : Fasting on the Day of Ashura (the 10th of Muharram).

Yahya related to me from Malik from Ibn Shihab that Humayd ibn Abd ar-Rahman ibn Awf heard Muawiya ibn Abi Sufyan say from the mimbar on the day of Ashura in the year in which he made the hajj, “People of Madina, where are your learned men? I heard the Messenger of Allah, may Allah bless him and grant him peace, say about this day, ‘This is the day of Ashura, and fasting it has not been prescribed for you. I am fasting it, and whoever of you wants to fast it can do so, and whoever does not want to, does not have to.'”