(69) باب صِيَامِ يَوْمِ عَاشُورَاءَ
Narrated `Aisha:
Allah's Messenger (ﷺ) ordered (the Muslims) to fast on the day of 'Ashura', and when fasting in the month of Ramadan was prescribed, it became optional for one to fast on that day ('Ashura') or not.
Reference: Sahih al-Bukhari 2001
In-book reference: Book 30, Hadith 106
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 3, Book 31, Hadith 219