Ash-Shama'il Al-Muhammadiyah 308
A’isha said:
"‘Ashura' was a day on which Quraish used to fast in the pagan era [al-jahiliyya], and Allah’s Messenger (Allah bless him and give him peace) used to keep it as well. Hence, when he arrived in Medina, he kept its fast and commanded its observance. Then, when Ramadan was decreed, [the fast of] Ramadan became the obligatory religious duty, and 'Ashura' was omitted, so one is free to fast this day or not to fast it.”
حَدَّثَنَا هَارُونُ بْنُ إِسْحَاقَ الْهَمْدَانِيُّ، قَالَ‏:‏ حَدَّثَنَا عَبْدَةُ بْنُ سُلَيْمَانَ، عَنْ هِشَامِ بْنِ عُرْوَةَ، عَنْ أَبِيهِ، عَنْ عَائِشَةَ، قَالَتْ‏:‏ كَانَ عَاشُورَاءُ يَوْمًا تَصُومُهُ قُرَيْشٌ فِي الْجَاهِلِيَّةِ، وَكَانَ رَسُولُ اللهِ صلى الله عليه وسلم يَصُومُهُ، فَلَمَّا قَدِمَ الْمَدِينَةَ صَامَهُ وَأَمَرَ بِصِيَامِهِ، فَلَمَّا افْتُرِضَ رَمَضَانُ كَانَ رَمَضَانُ هُوَ الْفَرِيضَةُ وَتُرِكَ عَاشُورَاءُ، فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ‏.‏