Mukhtasar Bukhari

Umm al-Mu'minin Aisha Siddiqa (may Allah be pleased with her) says: (We) used to fast on the day of Ashura before the fasting of Ramadan was made obligatory, and it was a day on which the covering was placed over the Ka'bah. Then, when Allah Almighty made Ramadan obligatory, the Messenger of Allah (peace and blessings of Allah be upon him) said: "(Now) whoever wishes to fast on Ashura may do so, and whoever does not wish to fast may leave it."