Sahih al-Bukhari 7265
Narrated Salama bin Al-Akwa`:
Allah's Messenger (ﷺ) said to a man from the tribe of Al-Aslam, "Proclaim among your people (or the 
people) on the day of 'Ashura' (tenth of Muharram), 'Whosoever has eaten anything should fast for the 
rest of the day; and whoever has not eaten anything, should complete his fast.' "
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، حَدَّثَنَا سَلَمَةُ بْنُ الأَكْوَعِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لِرَجُلٍ مِنْ أَسْلَمَ ‏
"‏ أَذِّنْ فِي قَوْمِكَ ـ أَوْ فِي النَّاسِ ـ يَوْمَ عَاشُورَاءَ أَنَّ مَنْ أَكَلَ فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ، وَمَنْ لَمْ يَكُنْ أَكَلَ فَلْيَصُمْ ‏"‏‏.‏