From Sahih Bukhari (Hadith 7265):
Narrated Salama bin Al-Akwa`:Allah's Messenger (ﷺ) said to a man from the tribe of Al-Aslam, "Proclaim among your people (or the people) on the day of 'Ashura' (tenth of Muharram), 'Whosoever has eaten anything should fast for the rest of the day; and whoever has not eaten anything, should complete his fast
Reference: Bukhari 7265