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."
Sahih al-Bukhari, Vol. 6, Book of Prophetic Commentary on the Qur'an, Hadith 30
Sahih al-Bukhari, Book of Prophetic Commentary on the Qur'an, Hadith 30