Sahih Bukhari Volume 7, Book 68, Hadith Number 478.

Narrated By Abu ‘Ubaid : The freed slave of Ibn Azhar that he witnessed the Day of ‘Id-al-Adha with ‘Umar bin Al-Khattab. ‘Umar offered the ‘Id prayer before the sermon and then delivered the sermon before the people, saying, “O people! Allah’s Apostle has forbidden you to fast (on the first day of) each of these two ‘Ida, for one of them is the Day of breaking your fast, and the other is the one, on which you eat the meat of your sacrifices.”