The Prophet ﷺ ordered a man from Aslam to announce to the people on the day of Ashura, "Whoever is fasting, let him complete his fast, and whoever has eaten, let him not eat anything more and let him complete his fast."
قال حدثنا حماد بن مسعدة عن يزيد يعني ابن أبي عبيد
أن النبي ﷺ أمر رجلا من أسلم أن يؤذن في الناس يوم عاشوراء من كان صائما فليتم صومه ومن كان أكل فلا يأكل شيئا وليتم صومه
Its chain of transmission is sahih. Hammad bin Masada is Tamimi Basri, the trustworthy one. The imams praised him, his hadith is found in the collections of all six, and he is well known. The hadith was mentioned earlier at 16064, and the man whom the Messenger of Allah sent is Asma bin Haritha, as mentioned in 16662.