(22) باب النَّهْىِ عَنْ صَوْمِ، يَوْمِ الْفِطْرِ وَيَوْمِ الأَضْحَى
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).
Reference: Sahih Muslim 827 c
In-book reference: Book 13, Hadith 180
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2536