Messenger of Allah ﷺ ordered (the Muslims) to fast on the day of 'Ashura', and when fasting in the month of Ramadan was prescribed, it became optional for one to fast on that day ('Ashura') or not.  

كَانَ رَسُولُ اللَّهِ ﷺ أَمَرَ بِصِيَامِ يَوْمِ عَاشُورَاءَ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ  

'A'isha (Allah be pleased with her) reported that the Messenger of Allah ﷺ had ordered to observe fast (on 'Ashura) before the fasting in Ramadan was made obligatory. But when it became obligatory, then he who wished fasted on the day of Ashura, and he who wished did not observe it (on that day).  

كَانَ رَسُولُ اللَّهِ ﷺ يَأْمُرُ بِصِيَامِهِ قَبْلَ أَنْ يُفْرَضَ رَمَضَانُ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَ يَوْمَ عَاشُورَاءَ 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