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.

Narrated Ibn Abbas (may Allah be pleased with them) that when the Prophet (peace and blessings of Allah be upon him) came to Madinah, he saw the Jews fasting on the day of Ashura. So he (peace be upon him) asked them: "What is this (reason that you fast on the day of Ashura)?" They said: "This is a great day; it is the day on which Allah saved the Children of Israel from their enemy, so Musa (peace be upon him) used to fast on this day." So the Prophet (peace be upon him) said: "I am more entitled to Musa (peace be upon him) than you." So he (peace be upon him) fasted on that day and ordered (the Muslims) to fast on it.