Mu’awiyah ibn Abu Sufyan (Radiyallahu 'anh) relates: I heard the Messenger of Allah (Subhanahu wa Ta'ala) say: "It is the day of ‘Ashura. Allah (Subhanahu wa Ta'ala) has not made fasting obligatory for you. But I am fasting. He who likes to observe fast among you should do so, and he who likes not to observe it (does not have to) observe it." [Sahih Muslim]

Abu Qatada (Radiyallahu 'anh) relates that the Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) said that the fast on the 10th of Muharram atones for the sins of the preceding year. [Sahih Muslim]

Abu Huraira (Radiyallahu 'anh) reports that the Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) said that after Ramadan, the fasts of Muharram have the greatest excellence. [Sahih Muslim]

Alhamdulillah, Allah (Subhanahu wa Ta'ala) has blessed us to see another year. The first month of this year is Muharram. In this month is an excellent day—the day of ‘Ashura—which falls on the 10th of Muharram. The Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) recommended that we fast on this day, by his Sunnah. He also indicated how we should observe the fast of ‘Ashura.

Hakam ibn Al-Arat (Radiyallahu 'anh) relates: I went to Ibn Abbas (Radiyallahu 'anh)… I said to him: Tell me about fasting on ‘Ashura. He said, "When you seen the new moon of Muharram count the (days) and (begin to) observe fast on the 9th." I said to him: "Is it how the Holy Messenger of Allah (Sall Allahu alaihi wa Aalihi wa Sallim) observed the fast?" He said, "Yes." [Sahih Muslim]

Hazrat Ibn Abbas (Radiyallahu 'anh) relates that when the Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) said: "If I survive till next year, I will definitely observe fast on the 9th of Muharram (as well)." [Sahih Muslim]

(Note: What the Holy Nabi (Sall Allahu alaihi wa Aalihi wa Sallim) meant was that he would also fast on the 9th as well as the 10th to which he was accustomed. We also should try fasting on the 9th and the 10th of Muharram.)