Was fasting the Day of Ashura obligatory at some point?

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There is a difference of opinion on whether fasting Ashura was obligatory. One view is that it was not obligatory, supported by the fact that the Prophet, peace be upon him, commanded only those who had not eaten to fast that day, and that intention at night is a condition for an obligatory act. Furthermore, no command for making up the fast was given to those who ate. This is supported by the sound hadith from Mu'awiyah where the Prophet stated: "Indeed, this is the Day of Ashura. Allah has not obligated its fast upon you, so whoever wills may fast it, and whoever wills may break his fast." However, it is narrated from Ahmad that it was obligatory because the Prophet commanded its fast, and when the fast of Ramadan became obligatory, Ashura became voluntary, as supported by a sound hadith from 'Aishah.