وَعَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ قَالَ: نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْ صَوْمِ يَوْمِ الْفِطْرِ وَالنَّحْرِ

Abu Sa'id al-Khudri said God’s messenger forbade fasting on the day of breaking the fast of Ramadan and on the day of sacrifice. (Bukhari and Muslim.)