Abu Sa'id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) forbade to observe fast on two days the day of Fitr and the day of Sacrifice ('Id-ul-Adha).
Sahih Muslim Book 13, Hadith 180
← Previous Hadith I heard from him (Abu Sa'id) a hadith which impressed me, and I said to him: Did you hear it from the Messenger of Allah (ﷺ)? Thereupon he said: (Is it possible) that (I should) say about the Messenger of Allah (ﷺ) that which I have not heard? I heard him saying: It is not proper to fast on two days, Adha and Fitr (at the end) of Ramadan.

Sahih Muslim Book 13, Hadith 179 Next Hadith → I had taken a vow that I would fast on the day (but it accidentally) synchronises with the day of Adha or the day of Fitr. Thereupon Ibn 'Umar (Allah be pleased with him) said: Allah, the Exalted, has commanded fulfilling of the vow, but the Messenger of Allah (ﷺ) has forbidden the observance of fast on this day.

Sahih Muslim Book 13, Hadith 181