Musnad Ahmad 27:754

Yahya narrated to us, from Ubaydullah, Nafi’ informed me, from Ibn Umar, who said: The day of Ashura was a day that the people of the Jahiliyyah (pre-Islamic era) used to fast. When Ramadan was revealed, the Messenger of Allah (peace and blessings of Allah be upon him) was asked about it. He said: "It is a day from the days of Allah the Exalted; whoever wishes may fast it, and whoever wishes may leave it."