Bukhari :: Book 2 :: Volume 26 :: Hadith 662
The people used to fast on 'Ashura (the tenth day of the month of Muharram) before the fasting of Ramadan was made obligatory. And on that day the Ka'ba used to be covered with a cover. When Allah made the fasting of the month of Ramadan compulsory, Allah's Apostle said, "Whoever wishes to fast (on the day of 'Ashura') may do so; and whoever wishes to leave it can do so."
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