From Sahih Bukhari (Hadith 1924):
Narrated Salama bin Al-Akwa`:Once the Prophet (ﷺ) ordered a person on 'Ashura' (the tenth of Muharram) to announce, "Whoever has eaten, should not eat any more, but fast, and who has not eaten should not eat, but complete his fast (till the end of the day)
Reference: Bukhari 1924