Sahih Muslim Book - 13, Hadith - 2644

وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا لَيْثٌ ح وَحَدَّثَنَا ابْنُ رُمْحٍ أَخْبَرَنَا اللَّيْثُ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ رضى الله عنهما أَنَّهُ ذُكِرَ عِنْدَ رَسُولِ اللَّهِ -صلى الله عليه وسلم- يَوْمُ عَاشُورَاءَ فَقَالَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- « كَانَ يَوْمًا يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَمَنْ أَحَبَّ مِنْكُمْ أَنْ يَصُومَهُ فَلْيَصُمْهُ وَمَنْ كَرِهَ فَلْيَدَعْهُ ».

It was narrated from Ibn Umar [may Allah be pleased with them] that the day of Ashura was mentioned in the presence of the Messenger of Allah (SAW). The Messenger of Allah said: “It was a day that was fasted by the people of Jahiliyyah. Whoever among you wants to fast, let him do so, and whoever does not want to, let him leave it.”