Sahih Muslim 1127
Sahih Muslim · Book 13: The Book of Fasting · Fasting on the day of Ashura'
'Alqama reported that Ash'ath b. Qais went to Ibn Mas'udd while he was eating on the day of Ashura. Thereupon he said:
وَحَدَّثَنِي مُحَمَّدُ بْنُ حَاتِمٍ ، حَدَّثَنَا إِسْحَاقُ بْنُ مَنْصُورٍ ، حَدَّثَنَا إِسْرَائِيلُ ، عَنْ مَنْصُورٍ ، عَنْ إِبْرَاهِيمَ ، عَنْ عَلْقَمَةَ ، قَالَ دَخَلَ الأَشْعَثُ بْنُ قَيْسٍ عَلَى ابْنِ مَسْعُودٍ وَهُوَ يَأْكُلُ يَوْمَ عَاشُورَاءَ فَقَالَ يَا أَبَا عَبْدِ الرَّحْمَنِ إِنَّ الْيَوْمَ يَوْمُ عَاشُورَاءَ . فَقَالَ قَدْ كَانَ يُصَامُ قَبْلَ أَنْ يَنْزِلَ رَمَضَانُ فَلَمَّا نَزَلَ رَمَضَانُ تُرِكَ فَإِنْ كُنْتَ مُفْطِرًا فَاطْعَمْ .
Abu Abd al-Rahman, it is the day of 'Ashura (and you are eating). Upon this he said: Fast was observed on (this day) before the (fasting) in Ramadan was made obligatory, but when it was made obligatory, (fasting on the day of 'Ashura) was abandoned. So if you are not fasting, then take food.