Sahih Bukhari
Accepting Information Given by a Truthful Person
Volume: 009 / Book Number: 091 / Hadith Number: 370
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.' "