The prohibition of fasting the days of At-Tashriq, which are the days of eating, drinking and remembering Allah, The Mighty and Sublime Sahih Muslim 1142 a

Sahih Muslim 1142a
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Translation
Ibn Ka'b b. Malik reported on the authority of his father that the Messenger of Allah (ﷺ) sent him and Aus b. Hadathan during the days of Tashriq to make this announcement

None but the believer would be admitted into Paradise, and the days of Mina' are the days meant for eating and drinking.