Language:
كتاب الصوم
30  
Fasting
(69)
Chapter: Observing Saum (fast) on the day of 'Ashura.
(69)
باب صِيَامِ يَوْمِ عَاشُورَاءَ
Sahih al-Bukhari 2007

Narrated Salama bin Al-Akwa`:

The Prophet (ﷺ) ordered a man from the tribe of Bani Aslam to announce amongst the people that whoever had eaten should fast the rest of the day, and whoever had not eaten should continue his fast, as that day was the day of 'Ashura' .

حَدَّثَنَا الْمَكِّيُّ بْنُ إِبْرَاهِيمَ، حَدَّثَنَا يَزِيدُ، عَنْ سَلَمَةَ بْنِ الأَكْوَعِ ـ رضى الله عنه ـ قَالَ أَمَرَ النَّبِيُّ صلى الله عليه وسلم رَجُلاً مِنْ أَسْلَمَ أَنْ أَذِّنْ فِي النَّاسِ ‏ "‏ أَنَّ مَنْ كَانَ أَكَلَ فَلْيَصُمْ بَقِيَّةَ يَوْمِهِ، وَمَنْ لَمْ يَكُنْ أَكَلَ فَلْيَصُمْ، فَإِنَّ الْيَوْمَ يَوْمُ عَاشُورَاءَ ‏"‏‏.‏
Reference : Sahih al-Bukhari 2007
In-book reference : Book 30, Hadith 112
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 225
  (deprecated numbering scheme)