(1) باب وُجُوبِ صَوْمِ رَمَضَانَ
Narrated `Aisha:
(The tribe of) Quraish used to fast on the day of Ashura' in the Pre-Islamic period, and then Allah's Apostle ordered (Muslims) to fast on it till the fasting in the month of Ramadan was prescribed; whereupon the Prophet (ﷺ) said, "He who wants to fast (on 'Ashura') may fast, and he who does not want to fast may not fast."
Reference: Sahih al-Bukhari 1893
In-book reference: Book 30, Hadith 3
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 117