Sahih al-Bukhari, Hadith 2007 ⓘ

Narrated Salama bin Al-Akwa`: The Prophet 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 ‏" ‏‏.‏