Abu Huraira (Allah be pleased with him) reported that the Messenger of Allah ﷺ forbade fasting on these two days. 'Id-ul-Adha and 'Id-ul-Fitr. (Using translation from Muslim 1138)
أَنَّ رَسُولَ اللهِ ﷺ «نَهَى عَنْ صِيَامِ يَوْمَيْنِ يَوْمِ الْفِطْرِ وَيَوْمِ الْأَضْحَى»