عَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كَانَ يَوْمُ عَاشُورَاءَ يَوْمًا يَصُومُهُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ فِي الْجَاهِلِيَّةِ، وَكَانَتْ قُرَيْشٌ تَصُومُهُ فِي الْجَاهِلِيَّةِ، فَلَمَّا قَدِمَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ، فَلَمَّا نَزَلَ رَمَضَانُ كَانَ رَمَضَانُ هُوَ الْفَرِيضَةَ وَتُرِكَ عَاشُورَاءُ
Lady Aisha (may Allah be pleased with her) says: The Messenger of Allah (peace and blessings be upon him) used to fast on the Day of Ashura during the Age of Ignorance, and the Quraysh also used to fast on this day during the Age of Ignorance. When the Noble Prophet (peace and blessings be upon him) came to Madinah, he also fasted on this day there and ordered the people to fast on this day as well. But when the fasting of the month of Ramadan was made obligatory, those fasts became obligatory and the fasting of the Day of Ashura was abandoned.