It is narrated from Sayyiduna Muhammad bin Sayfi (may Allah be pleased with him) that on the day of Ashura the Prophet (peace and blessings be upon him) ordered his caller to announce: "Whoever is fasting, let him continue his fast; and whoever has eaten and drunk, let him also complete his fast."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16581