Ibn `Abbas (May Allah be pleased with them) reported:
The Messenger of Allah (Sallallahu Alaihi Wasallam)
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 (Sallallahu Alaihi Wasallam)
emigrated from Makkah to Al-Madinah, he (Sallallahu Alaihi Wasallam)saw that
the Jews were fasting on this day. He (Sallallahu Alaihi Wasallam) 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 (Sallallahu Alaihi Wasallam) 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 (Sallallahu Alaihi Wasallam) 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 (Sallallahu Alaihi Wasallam)
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,).
Thus observance of two Saum on this occasion is a Sunnah of the Prophet (PBUH).
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 (PBUH) and instead observe the baseless practices of mourning
assemblies (Majalis), which is a grave sin practised by Shi`ah