(21) باب إِذَا نَوَى بِالنَّهَارِ صَوْمًا
Narrated Salama bin Al-Akwa`:
Once the Prophet (ﷺ) ordered a person on 'Ashura' (the tenth of Muharram) to announce, "Whoever has eaten, should not eat any more, but fast, and who has not eaten should not eat, but complete his fast (till the end of the day).
Reference: Sahih al-Bukhari 1924
In-book reference: Book 30, Hadith 33
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 147