Bajjah bin Abdullah narrates from his father that one day the Prophet (peace be upon him) said to him: "Today is the day of Ashura, so you should fast today." Upon hearing this, a man from Banu Amr bin Awf said: O Messenger of Allah! I am coming from my people in such a state that some of them were fasting and some were not. The Prophet (peace be upon him) said: "Go to them and convey this message: whoever among them is not fasting should spend the rest of the day without eating or drinking."
Hadith Referenceمسند احمد / من مسند القبائل / 27646