Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))

Bukhari :: Book 6 :: Volume 60 :: Hadith 28

Narrated Ibn 'Umar:

Fasting was observed on the day of 'Ashura' (i.e. 10th of Muharram) by the people of the Pre-lslamic Period. But when (the order of compulsory fasting) in the month of Ramadan was revealed, the Prophet said, "It is up to one to fast on it (i.e. day of 'Ashura') or not."