كتاب الصيام
13
The Book of Fasting
(19)
Chapter: Fasting on the day of Ashura'
(19)
باب صَوْمِ يَوْمِ عَاشُورَاءَ ‏‏
Sahih Muslim 1126 c

Ibn 'Umar (Allah be pleased with them) said that the day of 'Ashura was mentioned before the Messenger of Allah (may peace he upon him). Thereupon the Messenger of Allah, (ﷺ) said:

That was a day on which the people of pre-Islamic days used to observe fast. So he who amongst you likes to observe fast should do so, and he who does not like it should abandon it.
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 ‏"‏ ‏.‏
Reference : Sahih Muslim 1126 c
In-book reference : Book 13, Hadith 150
USC-MSA web (English) reference : Book 6, Hadith 2506
  (deprecated numbering scheme)