Narrated 'Aisha:
(The tribe of) Quraish used to fast on the day of Ashura' in the Pre-lslamic period, and then Allah's Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet said, "He who wants to fast (on 'Ashura') may fast, and he who does not want to fast may not fast."***
***It is permissible to fast the day of ‘Ashoora’ on its own, but it is better to fast the day before it or the day after it. This is the Sunnah that is proven from the Prophet (peace and blessings of Allaah be upon him), as he said, “If I am still alive next year, I will certainly fast the ninth.” Narrated by Muslim
***Muslim,Book 006, Number 2529:
Abdullah b 'Abbas reported that the Messenger of Allah (may peace be upon him) had said: If I live till the next (year), I would definitely observe fast on the 9th, and the narration transmitted by Abu Bakr is:" He meant the day of Ashura.***
***according to a version narrated by Muslim, “This is a great day when Allaah saved Moosa and his people and drowned Pharaoh and his people.***