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 4502

Narrated `Aisha:

The people used to fast on the day of 'Ashura' before fasting in Ramadan was prescribed but when (the order of compulsory fasting in) Ramadan was revealed, it was up to one to fast on it (i.e. 'Ashura') or not.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا ابْنُ عُيَيْنَةَ، عَنِ الزُّهْرِيِّ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ ـ رضى الله عنها ـ كَانَ عَاشُورَاءُ يُصَامُ قَبْلَ رَمَضَانَ، فَلَمَّا نَزَلَ رَمَضَانُ قَالَ ‏ "‏ مَنْ شَاءَ صَامَ، وَمَنْ شَاءَ أَفْطَرَ ‏"‏‏.‏
Reference : Sahih al-Bukhari 4502
In-book reference : Book 65, Hadith 29
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 29
  (deprecated numbering scheme)