Sahih al-Bukhari 2007
Sahih al-Bukhari · Book 30: Fasting · Observing Saum (fast) on the day of 'Ashura.
Narrated Salama bin Al-Akwa`:
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' .