Sayyiduna Muhammad bin Sayfi al-Ansari (may Allah be pleased with him) said: On the day of Ashura the Messenger of Allah (peace and blessings be upon him) came out to us and said: "Has anyone among you eaten anything today?" We said: Some have eaten and some have not eaten. He (peace and blessings be upon him) said: "Whoever has not eaten, let him complete his fast; and whoever has eaten something, let him complete the rest of his day." Then he (peace and blessings be upon him) sent word to the people of the countryside that they should do the same.
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16579