Sahih al-Bukhari 2007Book 30, Hadith 112
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 ".
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' .