(4) باب مَا كَانَ يَبْعَثُ النَّبِيُّ صلى الله عليه وسلم مِنَ الأُمَرَاءِ وَالرُّسُلِ وَاحِدًا بَعْدَ وَاحِدٍ
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: Sahih al-Bukhari 7265
In-book reference: Book 95, Hadith 19
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 9, Book 91, Hadith 370