It is mentioned in Bukhari and Muslim from Ibn Abbas (Allah be pleased with him and his father) that he was asked about fasting the Day of `Ashura [10th of Muharram]. He said, “I did not see the Messenger of Allah (blessings and peace be upon him) fast a day while more avid to seek its virtue than this day,” [meaning the Day of `Ashura]. [Bukhari (2006), and Muslim (1132)].

The Day of `Ashura has great virtue, and tremendous sanctity (hurma). The virtue of fasting it was known among the Prophets (peace be upon them all). Both Prophet Nuh and Prophet Musa (peace be upon them both) fasted it.

The Prophet (Allah bless him & give him peace) used to fast this day even in Mecca, though he had not yet ordered others to do so, as mentioned in both Bukhari and Muslim. [Bukhari (2002), Muslim (1125)]

When he migrated to Medina, and found the People of the Book fasting this day and venerating it, he ordered the Muslims to fast it, and encouraged it so much that even the children would fast it.

It has been reported in both Bukhari and Muslim from Ibn Abbas (Allah be pleased with him), that, When the Messenger of Allah (blessings and peace be upon him) reached Medina, he found the Jews fasting the Day of `Ashura, so he asked them, “What is this day you are fasting?” They said, “This is a tremendous day. Allah saved Musa and his people on this day and drowned Pharaoh and his people. Musa fasted it out of thanks, so we fast it too.” The Messenger of Allah (blessings and peace be upon him) said, “And we are more deserving of Musa than you are.” So he fasted this day, and ordered that it be fasted. [Bukhari (2004) and Muslim (1130)]

At the end of his life, the Messenger of Allah (blessings and peace be upon him) made the determination not to fast this day alone, but with another day [f: either before or after it], in order to be different from the People of the Book.

It has been reported in the Sahih of Imam Muslim (Allah have mercy on him), also from Ibn Abbas (Allah be pleased with him) that, “When the Messenger of Allah (blessings and peace be upon him) fasted the Day of `Ashura and ordered his companions to fast it, they said, ‘O Messenger of Allah! This is a day that the Jews and Christians venerate.’ So the Messenger of Allah (blessings and peace be upon him) said, ‘When next year comes – if Allah wills – we will fast the Ninth [of Muharram with it].’ But the next year did not come before the Messenger of Allah (blessings and peace be upon him) passed away.” [Muslim (1134), Abu Dawud (2445)]

And it is reported in the Musnad of Imam Ahmad (Allah have mercy on him), from Ibn Abbas (Allah be pleased with him) that the Prophet (Allah bless him and give him peace) said, “Fast the Day of `Ashura” and be different from the Jews by fasting a day before it or a day after it.” [Ahmad]