From Sahih Bukhari (Hadith 2007):
Narrated Salama bin Al-Akwa`:The Prophet (ﷺ) ordered a man from the tribe of Bani Aslam to announce amongst the people that whoever had eaten should fast the rest of the day, and whoever had not eaten should continue his fast, as that day was the day of 'Ashura
Reference: Bukhari 2007