The Messenger of Allah (ﷺ) forbade fasting on the Day of Fitr and the Day of Adha.
Sunan Ibn Majah Book 7, Hadith 84
← Previous Hadith The Messenger of Allah (ﷺ) delivered a sermon on the days of Tashriq (11th, 12th, and 13th of Dhul-Hijjah) and said: “No one will enter Paradise but a Muslim soul, and these days are the days of eating and drinking.”

Sunan Ibn Majah Book 7, Hadith 83 Next Hadith → “I was present for ‘Eid with ‘Umar bin Khattab. He started with the prayer before the sermon, and said: ‘The Messenger of Allah (ﷺ) forbade fasting on these two days, the Day of Fitr and the Day of Adha. As for the Day of Fitr, it is the day when you break your fast, and on the Day of Adha you eat the meat of your sacrifices.’”

Sunan Ibn Majah Book 7, Hadith 85