Quraish used to fast on the day of 'Ashura' in the Pre-Islamic period, and Messenger of Allah ﷺ too, used to fast on that day. When he came to Medina, he fasted on that day and ordered others to fast, too. Later when the fasting of the month of Ramadan was prescribed, he gave up fasting on the day of 'Ashura' and it became optional for one to fast on it or not.  

كَانَ يَوْمُ عَاشُورَاءَ تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ وَكَانَ رَسُولُ اللَّهِ ﷺ يَصُومُهُ فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ فَلَمَّا فُرِضَ رَمَضَانُ تَرَكَ يَوْمَ عَاشُورَاءَ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-lslamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not."  

كَانَ عَاشُورَاءُ يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَلَمَّا نَزَلَ رَمَضَانُ قَالَ مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ لَمْ يَصُمْهُ  

During the Pre-lslamic Period of ignorance the Quraish used to observe fasting on the day of 'Ashura', and the Prophet ﷺ himself used to observe fasting on it too. But when he came to Medina, he fasted on that day and ordered the Muslims to fast on it. When (the order of compulsory fasting in ) Ramadan was revealed, fasting in Ramadan became an obligation, and fasting on 'Ashura' was given up, and who ever wished to fast (on it) did so, and whoever did not wish to fast on it, did not fast.  

كَانَ يَوْمُ عَاشُورَاءَ تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ وَكَانَ النَّبِيُّ ﷺ يَصُومُهُ فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ فَلَمَّا نَزَلَ رَمَضَانُ كَانَ رَمَضَانُ الْفَرِيضَةَ وَتُرِكَ عَاشُورَاءُ فَكَانَ مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ لَمْ يَصُمْهُ  

He who wishes to observe this fast may do so, and he who wishes to abandon it may do so.  

كَانَتْ قُرَيْشٌ تَصُومُ عَاشُورَاءَ فِي الْجَاهِلِيَّةِ وَكَانَ رَسُولُ اللَّهِ ﷺ يَصُومُهُ فَلَمَّا هَاجَرَ إِلَى الْمَدِينَةِ صَامَهُ وَأَمَرَ بِصِيَامِهِ فَلَمَّا فُرِضَ شَهْرُ رَمَضَانَ قَالَ مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

'A'isha (Allah be pleased with her) reported. In the pre-Islamic days fast was observed on the day of Ashura, but with the advent of Islam (its position was ascertained as that of a voluntary fast). Then he who wished to fast fasted, and he who liked to abandon it abandoned it.  

أَنَّ يَوْمَ عَاشُورَاءَ كَانَ يُصَامُ فِي الْجَاهِلِيَّةِ فَلَمَّا جَاءَ الإِسْلاَمُ مَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

He who wishes to fast should do so, and he who wishes to break it may do so.  

أَنَّ قُرَيْشًا كَانَتْ تَصُومُ عَاشُورَاءَ فِي الْجَاهِلِيَّةِ ثُمَّ أَمَرَ رَسُولُ اللَّهِ ﷺ بِصِيَامِهِ حَتَّى فُرِضَ رَمَضَانُ فَقَالَ رَسُولُ اللَّهِ ﷺ مَنْ شَاءَ فَلْيَصُمْهُ وَمَنْ شَاءَ فَلْيُفْطِرْهُ  

'Ashura is one of the days of Allah, so he who wished should observe fast and he who wished otherwise should abandon it.  

أَنَّ أَهْلَ الْجَاهِلِيَّةِ كَانُوا يَصُومُونَ يَوْمَ عَاشُورَاءَ وَأَنَّ رَسُولَ اللَّهِ ﷺ صَامَهُ وَالْمُسْلِمُونَ قَبْلَ أَنْ يُفْتَرَضَ رَمَضَانُ فَلَمَّا افْتُرِضَ رَمَضَانُ قَالَ رَسُولُ اللَّهِ ﷺ إِنَّ عَاشُورَاءَ يَوْمٌ مِنْ أَيَّامِ اللَّهِ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

Yahya related to me from Malik from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet ﷺ said, "The day of Ashura was a day the Quraysh used to fast in the jahiliyya, and the Messenger of Allah ﷺ used also to fast it during the jahiliyya. Then when the Messenger of Allah ﷺ came to Madina he fasted it and ordered that it be fasted. Then Ramadan was made obligatory, and that became the fard instead of Ashura, but whoever wanted to, fasted it, and whoever did not want to, did not fast it."  

عَنْ عَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّهَا قَالَتْ كَانَ يَوْمُ عَاشُورَاءَ يَوْمًا تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ وَكَانَ رَسُولُ اللَّهِ ﷺ يَصُومُهُ فِي الْجَاهِلِيَّةِ فَلَمَّا قَدِمَ رَسُولُ اللَّهِ ﷺ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ فَلَمَّا فُرِضَ رَمَضَانُ كَانَ هُوَ الْفَرِيضَةَ وَتُرِكَ يَوْمُ عَاشُورَاءَ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

'Ashura is one of the days of Allah, so he who wished should observe fast and he who wished otherwise should abandon it. (Using translation from Muslim 1126a)  

أَنَّ أَهْلَ الْجَاهِلِيَّةِ كَانُوا يَصُومُونَ يَوْمَ عَاشُورَاءَ وَأَنَّ رَسُولَ اللهِ ﷺ صَامَهُ وَالْمُسْلِمُونَ قَبْلَ أَنْ يُفْتَرَضَ رَمَضَانُ فَلَمَّا افْتُرِضَ رَمَضَانُ قَالَ رَسُولُ اللهِ ﷺ إِنَّ عَاشُورَاءَ يَوْمٌ مِنْ أَيَّامِ اللهِ تَعَالَى فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

[Machine] The Messenger of Allah ﷺ said: "This is the day of Ashura, which the Quraysh used to fast during the time of ignorance. So whoever among you wishes to fast on this day, let them fast, and whoever wishes to abandon it, let them abandon it." Ibn Umar used to observe this fast except when it coincided with his own fast.  

قَالَ رَسُولُ اللَّهِ ﷺ «هَذَا يَوْمُ عَاشُورَاءَ كَانَتْ قُرَيْشٌ تَصُومُهُ فِي الْجَاهِلِيَّةِ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَصُومَهُ فَلْيَصُمْهُ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَتْرُكَهُ فَلْيَتْرُكْهُ» وَكَانَ ابْنُ عُمَرَ لَا يَصُومُهُ إِلَّا أَنْ يُوَافِقَ صِيَامَهُ  

[Machine] The Messenger of Allah ﷺ mentioned the Day of Ashura and said, "The people of pre-Islamic ignorance used to observe fasting on this day, so whoever wishes may fast and whoever wishes may break his fast."  

ذَكَرَ رَسُولُ اللهِ ﷺ عَاشُورَاءَ فَقَالَ «كَانَ صَوْمَ أَهْلِ الْجَاهِلِيَّةِ فَمَنْ شَاءَ صَامَ وَمَنْ شَاءَ أَفْطَرَ»  

[Machine] "And that the Messenger of Allah ﷺ used to fast it, and the Muslims as well, before the obligation of Ramadan was revealed. But when Ramadan was made obligatory, the Messenger of Allah ﷺ said: 'Indeed, Ashura is a day from the days of Allah, so whoever wishes, let him fast it, and whoever wishes, let him leave it.'"  

وَأَنَّ رَسُولَ اللهِ ﷺ صَامَهُ وَالْمُسْلِمُونَ قَبْلَ أَنْ يُفْرَضَ رَمَضَانُ فَلَمَّا افْتُرِضَ رَمَضَانُ قَالَ رَسُولُ اللهِ ﷺ إِنَّ عَاشُورَاءَ يَوْمٌ مِنْ أَيَّامِ اللهِ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَ  

"عَنِ ابْن عُمَرَ قَالَ: كَان يَومُ عَاشُورَاءَ يَوْمًا يَصُومُهُ أَهْل الْجَاهِلِيَّةِ، فَلَمَّا فُرِضَ صَوْمُ رَمَضَانَ سُئِلَ عَنْهُ النَّبِىُّ ﷺ فَقَالَ: هُوَ يَوْم مِنْ أَيَّامِ الله، فَمَنْ شَاءَ صَامَهُ، وَمَنْ شَاءَ تَركَهُ".