Indeed, fasting the 9th and the 10th of Muharram is a Prophetic Sunnah. The Prophet (peace and blessings be upon him) said, “Fasting the day of `Ashura’, I hope that Allah will accept it as expiation for the previous year.” (Reported by Muslim). Ibn `Abbas also quotes the Prophet (peace and blessings be upon him) as saying, “If I live next year, I shall also fast on the 9th day.” (Reported by Ahmad)
Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author, states the following: `Abdullah ibn `Abbas (may Allah be pleased with him) said: When the Messenger of Allah (peace and blessings be upon him) fasted on the day of `Ashura’ and told the people to fast, they said, “O Messenger of Allah, this is a day that is venerated by the Jews and Christians.” The Messenger of Allah (peace and blessings be upon him) 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 (peace and blessings be upon him) had passed away. (Reported by Muslim). Al-Shafi`i and his companions, Ahmad, Ishaq, and others said: It is recommended to fast both the 9th and the 10th because the Prophet (peace and blessings be upon him) fasted the 10th and intended to fast the 9th.
Based on this, there are different ways of fasting `Ashura’, the least of which is to fast the 10th only, but it is better to fast the 9th as well. The more one fasts in Muharram, the better.