Sahih Muslim Book 6, Hadith Number 2512.

Qais b Sakan reported that al-Ash’ath b. Qais went to ‘Abdullah on the day of ‘Ashura while he was eating. He said: Abu Muhammad, come near and dine. Upon this he said: I am fasting. Thereupon he said: We used to observe fast and then (this practice) was abandoned.