Sahih Bukhari Volume 3, Book 30, Hadith 1925

حَدَّثَنَا أَبُو عَاصِمٍ، عَنْ يَزِيدَ بْنِ أَبِي عُبَيْدٍ، عَنْ سَلَمَةَ بْنِ الأَكْوَعِ ـ رضى الله عنه ـ أَنَّ النَّبِيَّ صلى الله عليه وسلم بَعَثَ رَجُلاً يُنَادِي فِي النَّاسِ، يَوْمَ عَاشُورَاءَ ‏"‏ أَنْ مَنْ أَكَلَ فَلْيُتِمَّ أَوْ فَلْيَصُمْ، وَمَنْ لَمْ يَأْكُلْ فَلاَ يَأْكُلْ ‏"‏‏

Narrated By Salama bin Al-Akwa : Once the Prophet 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).