Sahih al-Bukhari 1924
Sahih al-Bukhari · Book 30: Fasting · If the intention of Saum was made in the daytime.
Narrated Salama bin Al-Akwa`:
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 " .
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).