(69) باب صِيَامِ يَوْمِ عَاشُورَاءَ
Narrated Aisha:
Quraish used to fast on the day of 'Ashura' in the Pre-Islamic period, and Allah's Messenger (ﷺ) too, used to fast on that day. When he came to Medina, he fasted on that day and ordered others to fast, too. Later when the fasting of the month of Ramadan was prescribed, he gave up fasting on the day of 'Ashura' and it became optional for one to fast on it or not.
Reference: Sahih al-Bukhari 2002
In-book reference: Book 30, Hadith 107
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 220