Language:
كتاب أخبار الآحاد
95  
Accepting Information Given by a Truthful Person
(4)
Chapter: The Prophet (saws) used to send commanders and messengers one after another
(4)
باب مَا كَانَ يَبْعَثُ النَّبِيُّ صلى الله عليه وسلم مِنَ الأُمَرَاءِ وَالرُّسُلِ وَاحِدًا بَعْدَ وَاحِدٍ
Sahih al-Bukhari 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.' "

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، حَدَّثَنَا سَلَمَةُ بْنُ الأَكْوَعِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ لِرَجُلٍ مِنْ أَسْلَمَ ‏ "‏ أَذِّنْ فِي قَوْمِكَ ـ أَوْ فِي النَّاسِ ـ يَوْمَ عَاشُورَاءَ أَنَّ مَنْ أَكَلَ فَلْيُتِمَّ بَقِيَّةَ يَوْمِهِ، وَمَنْ لَمْ يَكُنْ أَكَلَ فَلْيَصُمْ ‏"‏‏.‏
Reference : Sahih al-Bukhari 7265
In-book reference : Book 95, Hadith 19
USC-MSA web (English) reference : Vol. 9, Book 91, Hadith 370
  (deprecated numbering scheme)