كتاب التفسير
65
Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
(24)
“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).
(24)
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.‏
Reference : Sahih al-Bukhari 4503
In-book reference : Book 65, Hadith 30
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 30
  (deprecated numbering scheme)