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  

'Ashurah was a day on which we used to fast in pre-Islamic days. When (fasting of) Ramadan was prescribed, the Messenger of Allah ﷺ said: This is one of the days of Allah ; he who wishes may fast on it.  

كَانَ عَاشُورَاءُ يَوْمًا نَصُومُهُ فِي الْجَاهِلِيَّةِ فَلَمَّا نَزَلَ رَمَضَانُ قَالَ رَسُولُ اللَّهِ ﷺ هَذَا يَوْمٌ مِنْ أَيَّامِ اللَّهِ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ  

"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".