Fasting

Bukhari :: Book 3 :: Volume 31 :: Hadith 219

Narrated 'Aisha:

Allah's Apostle 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.

Source materials are from the University of Southern California MSA site
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