Sahih Bukhari | Volume 6 | Hadith Number 31

Narrated by 'Aisha
During the Pre-Islamic Period of ignorance the Quraish used to observe fasting on the day of 'Ashura', and the Prophet himself used to observe fasting on it too. But when he came to Medina, he fasted on that day and ordered the Muslims to fast on it. When (the order of compulsory fasting in) Ramadan was revealed, fasting in Ramadan became an obligation, and fasting on 'Ashura' was given up, and who ever wished to fast (on it) did so, and whoever did not wish to fast on it, did not fast.