Sahih al-Bukhari 3831
Narrated `Aisha:
'Ashura' (i.e. the tenth of Muharram) was a day on which the tribe of Quraish used to fast in the prelslamic 
period of ignorance. The Prophet (ﷺ) also used to fast on this day. So when he migrated to 
Medina, he fasted on it and ordered (the Muslims) to fast on it. When the fasting of Ramadan was 
enjoined, it became optional for the people to fast or not to fast on the day of Ashura.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، قَالَ هِشَامٌ حَدَّثَنِي أَبِي، عَنْ عَائِشَةَ ـ رضى الله عنها ـ قَالَتْ كَانَ عَاشُورَاءُ يَوْمًا تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ، وَكَانَ النَّبِيُّ صلى الله عليه وسلم يَصُومُهُ، فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ، فَلَمَّا نَزَلَ رَمَضَانُ كَانَ مَنْ شَاءَ صَامَهُ، وَمَنْ شَاءَ لاَ يَصُومُهُ‏.‏