"Ashura was a day that the Quraish used to fast during Jahiliyyah, and the Messenger of Allah used to fast it. But when (the fast of) Ramadan became obligatory, the Ramadan was the required and Ashura was left. So whoever wanted to, he fasted it, and whoever wanted to, he left it."
Reference: Jami` at-Tirmidhi 753 In-book : Book 8, Hadith 72 English translation : Vol. 2, Book 3, Hadith 753
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