The Book of Fasting (Kitab Al-Sawm)
Muslim :: Book 6 : Hadith 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.