Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
Bukhari :: Book 6 :: Volume 60 :: Hadith 31
During the Pre-lslamic Period of ignorance the Quraish used to observe fasting on the day of 'Ashura', and the Prophet himself used to observe fasting on it too. But when he came to Medina, he fasted on that day and ordered the Muslims to fast on it. When (the order of compulsory fasting in ) Ramadan was revealed, fasting in Ramadan became an obligation, and fasting on 'Ashura' was given up, and who ever wished to fast (on it) did so, and whoever did not wish to fast on it, did not fast.
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