Narrated Salama bin Al-Akwa':

Allah's Apostle 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 ﷺ 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 ".