Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the people of the Age of Ignorance used to fast on the Day of Ashura (that is, the tenth of Muharram), and before the fasting of Ramadan was made obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) and other Muslims also used to fast. When Ramadan was made obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Ashura is one of Allah’s days; whoever wishes may fast on it, and whoever wishes may leave it."