Abu Sa'id Al-Khudri (RAA) narrated. 'The Messenger of Allah (ﷺ) has prohibited fasting on two days; the day of Fitr (breaking the fast of Ramadan) and on the day of sacrifice ('Idul Adha).' Agreed upon.
English reference: Book 5, Hadith 706
Arabic reference: Book 5, Hadith 686