Sahih Muslim
The Book of Fasting (Kitab Al-Sawm)
Book Number: 006 / Hadith Number: 2513
'Alqama reported that Ash'ath b. Qais went to Ibn Mas'udd while he was eating on the day of Ashura. Thereupon he said: Abu Abd al-Rahman, it is the day of 'Ashura (and you are eating). Upon this he said: Fast was observed on (this day) before the (fasting) in Ramadan was made obligatory, but when it was made obligatory, (fasting on the day of 'Ashura) was abandoned. So if you are not fasting, then take food.