Salama b. al-Akwa' (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) sent a person of the tribe of Aslam on the day of 'Ashura and commanded him to declare to the people to observe fast in case they had not observed it, and to complete fast till evening if they had taken food
حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا حَاتِمٌ، - يَعْنِي ابْنَ إِسْمَاعِيلَ - عَنْ يَزِيدَ بْنِ أَبِي، عُبَيْدٍ عَنْ سَلَمَةَ بْنِ الأَكْوَعِ، - رضى الله عنه - أَنَّهُ قَالَ بَعَثَ رَسُولُ اللَّهِ صلى الله عليه وسلم رَجُلاً مِنْ أَسْلَمَ يَوْمَ عَاشُورَاءَ فَأَمَرَهُ أَنْ يُؤَذِّنَ فِي النَّاسِ " مَنْ كَانَ لَمْ يَصُمْ فَلْيَصُمْ وَمَنْ كَانَ أَكَلَ فَلْيُتِمَّ صِيَامَهُ إِلَى اللَّيْلِ " .
Muslim
The Book of Fasting • Hadith 174