Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
Bukhari :: Book 6 :: Volume 60 :: Hadith 30
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."