“The Prophet (sal Allahu alaihi wa sallam) said: “Fasting the day of ‘Arafah I hope Allah will expiate thereby for the year before it and the year after it, and fasting the day of ‘Ashoora I hope Allah will expiate thereby for the year that came before it.” [Sahih Muslim]

It is recommended to fast the 9th of Muharram also as The Messenger of Allah (sal Allahu alaihi wa sallam) said, “Next year, if Allah wills, we will fast on the ninth day.” But by the time the following year came, the Messenger of Allah (sal Allahu alaihi wa sallam) had passed away.”