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  

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  

'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  

'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). (Using translation from Muslim 1125d)  

كَانَ رَسُولُ اللهِ ﷺ «يَأْمُرُ بِصِيَامِ عَاشُورَاءَ قَبْلَ أَنْ يُفْرَضَ رَمَضَانُ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَ عَاشُورَاءَ وَمَنْ شَاءَ أَفْطَرَ»