Sahih al-Bukhari 4502
Narrated `Aisha:
The people used to fast on the day of 'Ashura' before fasting in Ramadan was prescribed but when (the 
order of compulsory fasting in) Ramadan was revealed, it was up to one to fast on it (i.e. 'Ashura') or 
not.
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا ابْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ كَانَ عَاشُورَاءُ يُصَامُ قَبْلَ رَمَضَانَ، فَلَمَّا نَزَلَ رَمَضَانُ قَالَ ‏
"‏ مَنْ شَاءَ صَامَ، وَمَنْ شَاءَ أَفْطَرَ ‏"‏‏.‏