Sahih Muslim | Book 6 | Hadith Number 2536

Narrated by Abu Sa'id al-Khudri (Allah be pleased with him)
Abu Sa'id al-Khudri (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) forbade to observe fast on two days the day of Fitr and the day of Sacrifice ('Id-ul-Adha).