Imam Al-Nawawi's Riyad-us-Saliheen
Chapter 227
The Excellence of Observing Saum on the Day of 'Arafah, 'Ashura' and Tasu'a (i.e., 9th of Muharram)

1250. Abu Qatadah (May Allah be pleased with him) reported: The Messenger of Allah (sallallaahu ’alayhi wa sallam) was asked about the observance of Saum (fasting) on the day of `Arafah. He said, "It is an expiation for the sins of the preceding year and the current year.''
[Muslim].

Commentary:  The ninth day of Dhul-Hijjah is called Yaum-`Arafah . On this day, pilgrims stop and stand in prayers at `Arafat which is the most important ritual of Hajj, so much so that Hajj is deemed incomplete without it. On that day, the pilgrims are engaged in the remembrance of Allah and prayers as that is their most important worship on that day. For this reason, Saum is not desirable for them, but for other people, Saum has a special merit. The importance and merit of this Saum can be judged from the fact that it expiates sins for two consecutive years: the previous year and the current year. But these sins relate only to the minor sins, not the major ones, or become a means of elevation of one's status.

1251. 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:  `Ashura' 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. It is regrettable indeed that present-day Muslims do not follow this Sunnah of the Prophet (sallallaahu ’alayhi wa sallam) and instead observe the baseless practices of mourning assemblies (Majalis), which is a grave sin practised by Shi`ah.

1252. 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].

1253. 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].

Commentary: Some people think these words stand to mean that `he will observe Saum on the 9th of Muharram only. But this interpretation does not agree with other Ahadith. In order to oppose the Jews, he (sallallaahu ’alayhi wa sallam) resolved to observe one more Saum with that of 10th of Muharram and ordained it for the Muslims. In this connection, we have quoted above the authority from Musnad Ahmad. Hence, the meanings we have given in the translation of the text are correct.