English Translation
Narrated 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.' "
This hadith is from Sahih al-Bukhari, the most rigorously authenticated collection in Islamic scholarship. No editorial opinion is added.