Sahih Muslim 1126
Sahih Muslim · Book 13: The Book of Fasting · Fasting on the day of Ashura'
'Abdullah b. Umar (Allah be pleased with them) reported that the day of 'Ashura was mentioned before the Messenger of Allah (ﷺ) and he said:
وَحَدَّثَنَا أَحْمَدُ بْنُ عُثْمَانَ النَّوْفَلِيُّ ، حَدَّثَنَا أَبُو عَاصِمٍ ، حَدَّثَنَا عُمَرُ بْنُ مُحَمَّدِ بْنِ، زَيْدٍ الْعَسْقَلاَنِيُّ حَدَّثَنَا سَالِمُ بْنُ عَبْدِ اللَّهِ ، حَدَّثَنِي عَبْدُ اللَّهِ بْنُ عُمَرَ ، - رضى الله عنهما - قَالَ ذُكِرَ عِنْدَ رَسُولِ اللَّهِ صلى الله عليه وسلم يَوْمُ عَاشُورَاءَ فَقَالَ " ذَاكَ يَوْمٌ كَانَ يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَمَنْ شَاءَ صَامَهُ وَمَنْ شَاءَ تَرَكَهُ " .
It is a day when the people in the pre-Islamic days need to observe fast, so he who wishes to observe fast should do so, and he who wishes to abandon it should do so.