كتاب الصيام
13
The Book of Fasting
(21)
Chapter: Whoever eats on'Ashura, let him refrain (from eating) for the rest of the day
(21)
باب مَنْ أَكَلَ فِي عَاشُورَاءَ فَلْيَكُفَّ بَقِيَّةَ يَوْمِهِ ‏‏
Sahih Muslim 1135

Salama b. al-Akwa' (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) sent a person of the tribe of Aslam on the day of 'Ashura and commanded him to declare to the people to observe fast in case they had not observed it, and to complete fast till evening if they had taken food

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا حَاتِمٌ، - يَعْنِي ابْنَ إِسْمَاعِيلَ - عَنْ يَزِيدَ بْنِ أَبِي، عُبَيْدٍ عَنْ سَلَمَةَ بْنِ الأَكْوَعِ، - رضى الله عنه - أَنَّهُ قَالَ بَعَثَ رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً مِنْ أَسْلَمَ يَوْمَ عَاشُورَاءَ فَأَمَرَهُ أَنْ يُؤَذِّنَ فِي النَّاسِ ‏ "‏ مَنْ كَانَ لَمْ يَصُمْ فَلْيَصُمْ وَمَنْ كَانَ أَكَلَ فَلْيُتِمَّ صِيَامَهُ إِلَى اللَّيْلِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1135
In-book reference : Book 13, Hadith 174
USC-MSA web (English) reference : Book 6, Hadith 2530
  (deprecated numbering scheme)