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

Abdullah b. 'Umar (Allah be pleased with both of them) reported that he heard the Messenger of Allah (ﷺ) say about the day of Ashura:

It is a day on which the people of pre-Islamic days observed fast. So he who liked to fast on this day should do so, and he who liked to abandon it should abandon it. 'Abdullah (Allah be pleased with him) did not observe fast except when it coincided (with the days when he was in the habit of observing voluntary fasts during every month).
حَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو أُسَامَةَ، عَنِ الْوَلِيدِ، - يَعْنِي ابْنَ كَثِيرٍ - حَدَّثَنِي نَافِعٌ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، - رضى الله عنهما - حَدَّثَهُ أَنَّهُ، سَمِعَ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ فِي يَوْمِ عَاشُورَاءَ ‏ "‏ إِنَّ هَذَا يَوْمٌ كَانَ يَصُومُهُ أَهْلُ الْجَاهِلِيَّةِ فَمَنْ أَحَبَّ أَنْ يَصُومَهُ فَلْيَصُمْهُ وَمَنْ أَحَبَّ أَنْ يَتْرُكَهُ فَلْيَتْرُكْهُ ‏"‏ ‏.‏ وَكَانَ عَبْدُ اللَّهِ - رضى الله عنه - لاَ يَصُومُهُ إِلاَّ أَنْ يُوَافِقَ صِيَامَهُ ‏.‏
Reference : Sahih Muslim 1126 d
In-book reference : Book 13, Hadith 151
USC-MSA web (English) reference : Book 6, Hadith 2507
  (deprecated numbering scheme)