Ibn `Abbas (May Allah be pleased with them) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) observed Saum (fasting) on the day of `Ashura’ and commanded us to fast on this day. [Al-Bukhari and Muslim].
Commentary: `Ashoora’ 10th of Muharram. In another Hadith, it is stated that when the Prophet (sallallaahu ’alayhi wa sallam) emigrated from Makkah to Al-Madinah, he saw that the Jews were fasting on this day. He asked them why they did this on that day, and they told him that they did it for the reason that Allah granted Prophet Musa emancipation from Pharaoh on this day, and so they observed Saum in token of their joy. On this, he (sallallaahu ’alayhi wa sallam) said that in this happiness of Prophet Musa, Muslims have a greater right to observe Saum than the Jews. Consequently, he also observed Saum on 10th of Muharram. Then, he said that if he lived the following year, he would observe Saum on 9th of Muharram also so that they would make themselves different from the Jews. According to another Hadith, he (sallallaahu ’alayhi wa sallam) ordained the Muslims to observe Saum on `Ashura’ and in addition one more Saum on 9th or 11th of Muharram in opposition to the Jews.
(Musnad Ahmad, Vol.4, P-21, edited by Ahmad Shakir; and Mujamma` Az-Zawaid, Vol.3, P-188).
Thus observance of two Saum on this occasion is a Sunnah of the Prophet (sallallaahu ’alayhi wa sallam). These two Saum can be observed either on 9th and 10th or 10th and 11th of Muharram.
Abu Qatadah (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) was asked about observing As-Saum (the fast) on the tenth day of Muharram, and he replied, “It is an expiation for the sins of the preceding year.” [Muslim].
Ibn `Abbas (May Allah be pleased with them) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) said, “If I remain alive till the next year, I shall also observe fast on the ninth of Muharram.” [Muslim].