Mukhtasar Bukhari

Umm al-Mu'mineen Aisha Siddiqa (may Allah be pleased with her) says that during the era of ignorance, the Quraysh used to fast on the day of Ashura, and the Messenger of Allah (peace and blessings be upon him) also used to fast on this day. Then, when the Prophet (peace and blessings be upon him) arrived in Madinah, he also fasted on this day and ordered the people to do so as well. Then, when the fasting of Ramadan was made obligatory, the fasting of Ashura was abandoned; whoever wished, fasted on Ashura, and whoever wished, did not fast.