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.
عَنْ عَائِشَةَ ؓ كَانَ عَاشُورَاءُ يُصَامُ قَبْلَ رَمَضَانَ فَلَمَّا نَزَلَ رَمَضَانُ قَالَ مَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ
[Machine] That the Prophet ﷺ used to recommend fasting on the day of Ashura before the month of Ramadan was made obligatory. So when Ramadan was made obligatory, whoever wished fasted and whoever wished broke their fast.
أَنَّ النَّبِيَّ ﷺ كَانَ يَأْمُرُ بِصِيَامِ عَاشُورَاءَ قَبْلَ أَنْ يَنْزِلَ رَمَضَانُ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ