If it is asked: “How do you know that Moses fasted on it?”
We reply: It is established in the two collections of authentic hadith that when the Messenger of Allah (peace and blessings be upon him) asked them about it, they said: “It is a great day on which Allah saved Moses and his people and drowned Pharaoh and his people. Moses therefore fasted on it in gratitude, and so we fast on it.” The Messenger of Allah (peace and blessings be upon him) said: “We have a greater right to Moses and are closer to him than you are.” He therefore fasted on it and instructed others to fast on it. Since he accepted their account and did not declare it false, it is known that Moses fasted on it in gratitude to Allah. This added to the honor already accorded to the day before the Hijrah, reinforcing it further, until the Messenger of Allah (peace and blessings be upon him) sent a herald to announce among the Ansār (1) that they should fast on it and that anyone who had already eaten should abstain for the rest of the day. It appears that he made this binding and obligatory upon them, as will be established below, if Allah wills.
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As for the third difficulty—that the Messenger of Allah (peace and blessings be upon him) used to fast on ‘Āshūrā’ before fasting in Ramadan was prescribed, but abandoned it once fasting in Ramadan was prescribed—it can only be resolved by holding that fasting on ‘Āshūrā’ had been obligatory before Ramadan. What was abandoned, then, was the obligation to fast on it, not its recommended status. This is the only possible interpretation, for a year before his death, when he was told that the Jews fasted on it, he (peace and blessings be upon him) said: “If I live until next year, I will certainly fast on the ninth”—meaning, in addition to ‘Āshūrā’. He also said: “Differ from the Jews: fast a day before it and a day after it” (2)—meaning, in addition to it. There is no doubt that this occurred toward the end, whereas early on he liked to agree with the People of the Book in matters concerning which he had received no instruction. It is therefore known that fasting on it did not cease to be recommended.