Sahih Muslim Book 6, Hadith Number 2533.

Chapter : Forbiddance to observe fast on ‘Id-ul-Fitr and ‘Id-al-Adha.

Abu Ubaid, the freed slave of Ibn Azhar, reported: I observed Id along with Umar b. al-Khattab (Allah be pleased with him). He came (out in an open space) and prayed and (after) completing it addressed the people and said: The Messenger of Allah (may peace be upon him) has forbidden the observing of fast on these two days. One is the day of Fitr (at the end of your fasts), and the second one, the day when you eat (the meat) of your sacrifices.