The Messenger of Allah ﷺ prohibited fasting on two days, the day of Fitr and the day of Nahr.
حدثنا روح حدثنا مالك عن محمد بن يحيى بن حبان عن الأعرج
أن رسول الله ﷺ نهى عن صيام يومين يوم الفطر ويوم النحر
Its isnad is sahih. Bukhari related it in 10/24, no. 5571, in Fath al-Bari, in the book of sacrifices, chapter on what may be eaten of the meat of sacrifices. Muslim related it in 2/800, no. 1142, in the book of fasting, chapter on the prohibition of fasting on the two days of Eid. Abu Dawud related it, no. 2416, and Tirmidhi related it in 3/132, no. 772, both in the book of fasting.