Sayyiduna Asma' bin Harithah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) sent me on the day of Ashura and said: "Go to your people and command them to fast this day." I said: O Messenger of Allah, I think I will reach them only after they have eaten. He (peace and blessings be upon him) said: "Command whoever among them has eaten to fast the rest of his day."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 865