Sahih Bukhari Volume 003, Book 031, Hadith Number 211.

Narated By Abu 'Ubaid : (The slave of Ibn Azhar) I witnessed the 'Id with 'Umar bin Al-Kattab who said, Allah's Apostle has forbidden people to fast on the day on which you break fasting (the fasts of Ramadan) and the day on which you eat the meat of your sacrifices (the first day of 'Id ul Fitr and 'Id ul-Adha).