Ibrahim bin Abdullah bin Qariz (may Allah have mercy on him) said: I heard Mu'awiyah bin Abi Sufyan (may Allah be pleased with him) on the pulpit; he mentioned the day of Ashura and said: The Messenger of Allah (peace and blessings be upon him) said: "We used to fast the day of Ashura in the Jahiliyyah; then Allah made the fasting of Ramadan obligatory upon us and so removed the fasting of Ashura from us. So whoever wishes may fast it, and whoever wishes may leave it."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16855