Sahih Bukhari

Narrated `Abdullah:That Al-Ash'ath entered upon him while he was eating. Al-Ash'ath said, ""Today is 'Ashura."" I said (to him), ""Fasting had been observed (on such a day) before (the order of compulsory fasting in) Ramadan was revealed. But when (the order of fasting in) Ramadan was revealed, fasting (on 'Ashura') was given up, so come and eat