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).
عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ ؓ أَنَّ رَسُولَ اللَّهِ ﷺ نَهَى عَنْ صِيَامِ يَوْمَيْنِ يَوْمِ الْفِطْرِ وَيَوْمِ النَّحْرِ