Sahih al-Bukhari 4503
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · “O you who believe! Observing As-Saum (the fasting) is prescribed for you as it was prescribed for those before you that you, may become Al- Muttaqun (V.2:183).
Narrated `Abdullah:
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.
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."