Sahih Bukhari Book 31. Fasting
Narated By Ibn 'Umar : The Prophet observed the fast on the 10th of Muharram ('Ashura), and ordered (Muslims) to fast on that day, but when the fasting of the month of Ramadan was prescribed, the fasting of the 'Ashura' was abandoned. 'Abdullah did not use to fast on that day unless it coincided with his routine fasting by chance.