Chapter 30: Fasting
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."
Sahih al-Bukhari 1893