Sahih Bukhari Volume 9, Book 91, Hadith Number 370.

Narrated By Salama bin Al-Akwa’ : Allah’s Apostle 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.'”