The Prophet ﷺ ordered his announcer on the day of Ashura to proclaim that whoever had eaten in the morning should now refrain, and whoever had not eaten should complete his fast.
قال حدثنا صفوان بن عيسى قال أخبرنا يزيد يعني ابن أبي عبيد
أن النبي ﷺ أمر مناديه يوم عاشوراء أن من كان اصطبح فليمسك ومن لم يكن اصطبح فليتم صومه
No references are available.
Its isnad is sahih. Safwan bin Isa is Zuhri, Abu Muhammad, the Basran, the qassam, the well-known thiqah, and his hadith is found in Muslim. The hadith has already appeared at 16459.