Fasting

Bukhari :: Book 3 :: Volume 31 :: Hadith 117

Narrated 'Aisha:

(The tribe of) Quraish used to fast on the day of Ashura' in the Pre-lslamic 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 said, "He who wants to fast (on 'Ashura') may fast, and he who does not want to fast may not fast."