The fast of ‘Ashura was prescribed before the fasts of Ramadan. The Jews observed it and so did the people of Arabia before the dawn of Islam.
It is related by Imam Bukhari on the authority of Ibn-i-Abbas that when the Prophet 
came to Madinah he found that the Jews observed the fast of ‘Ashura. He enquired about it from them and was told that it was the day on which God had delivered the Children of Israel from the enemy and Moses used to keep a fast on it as an expression of gratitude to the Almighty. The Prophet thereupon, remarked that ‘Moses has a greater claim upon me than upon you,’ and he fasted on that day and instructed his followers to do the same. Read more... (2557 words, 2 images, estimated 10:14 mins reading time)