Sahih al-Bukhari 4503
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."
حَدَّثَنِي مَحْمُودٌ، أَخْبَرَنَا عُبَيْدُ اللَّهِ، عَنْ إِسْرَائِيلَ، عَنْ مَنْصُورٍ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ دَخَلَ عَلَيْهِ الأَشْعَثُ وَهْوَ يَطْعَمُ فَقَالَ الْيَوْمُ عَاشُورَاءُ‏.‏ فَقَالَ كَانَ يُصَامُ قَبْلَ أَنْ يَنْزِلَ رَمَضَانُ، فَلَمَّا نَزَلَ رَمَضَانُ تُرِكَ، فَادْنُ فَكُلْ‏.‏