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 1136 a

Rubayyi' daughter of Mu'awwidh b. 'Afra' said that the Messenger of Allah (ﷺ) sent (a person) on the morning of Ashura to the villages of Ansar around Medina (with this message):

He who got up in the morning fasting (without eating anything) he should complete his fast, and he who had had his breakfast in the morning, he should complete the rest of the day (without food). The Companions said; We henceforth observed fast on it (on the day of 'Ashura) and, God willing, made our children observe that. We went to the mosque and made toys out of wool for them and when anyone felt hungry and wept for food we gave them these toys till it was the time to break the fast.
وَحَدَّثَنِي أَبُو بَكْرِ بْنُ نَافِعٍ الْعَبْدِيُّ، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ بْنِ لاَحِقٍ، حَدَّثَنَا خَالِدُ، بْنُ ذَكْوَانَ عَنِ الرُّبَيِّعِ بِنْتِ مُعَوِّذِ بْنِ عَفْرَاءَ، قَالَتْ أَرْسَلَ رَسُولُ اللَّهِ صلى الله عليه وسلم غَدَاةَ عَاشُورَاءَ إِلَى قُرَى الأَنْصَارِ الَّتِي حَوْلَ الْمَدِينَةِ ‏ "‏ مَنْ كَانَ أَصْبَحَ صَائِمًا فَلْيُتِمَّ صَوْمَهُ وَمَنْ كَانَ أَصْبَحَ مُفْطِرًا فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ ‏"‏ ‏.‏ فَكُنَّا بَعْدَ ذَلِكَ نَصُومُهُ وَنُصَوِّمُ صِبْيَانَنَا الصِّغَارَ مِنْهُمْ إِنْ شَاءَ اللَّهُ وَنَذْهَبُ إِلَى الْمَسْجِدِ فَنَجْعَلُ لَهُمُ اللُّعْبَةَ مِنَ الْعِهْنِ فَإِذَا بَكَى أَحَدُهُمْ عَلَى الطَّعَامِ أَعْطَيْنَاهَا إِيَّاهُ عِنْدَ الإِفْطَارِ ‏.‏
Reference : Sahih Muslim 1136 a
In-book reference : Book 13, Hadith 175
USC-MSA web (English) reference : Book 6, Hadith 2531
  (deprecated numbering scheme)