Mukhtasar Bukhari

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.