“O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you that you, may become Al- Muttaqun (V.2:183). Sahih al-Bukhari 4502
Arabic hadith text
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 "
Translation
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.