Sahih Al Bukhari Hadith Number 1924

Narrated Salama bin Al-Akwa`: Once the Prophet (peace be upon him) 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).

30  Fasting
Hadith number 1924
Volume no 3
Chapter: 21 (If the intention of Saum was made in the daytime.)