Sahih Bukhari Volume 002, Book 015, Hadith Number 099.
Narated By Al-Bara' bin 'Azib : On the day of Nahr Allah's Apostle delivered the Khutba after the 'Id prayer and said, "Anyone who prayed like us and slaughtered the sacrifice like we did then he acted according to our (Nusuk) tradition of sacrificing, and whoever slaughtered the sacrifice before the prayer, then that was just mutton (i.e. not sacrifice)." Abu Burda bin Naiyar stood up and said, "O Allah's Apostle! By Allah, I slaughtered my sacrifice before I offered the (Id) prayer and thought that today was the day of eating and drinking (non-alcoholic drinks) and so I made haste (in slaughtering) and ate and also fed my family and neighbors." Allah's Apostle said, "That was just mutton (not a sacrifice)." Then Abu Burda said, "I have a young she-goat and no doubt, it is better than two sheep. Will that be sufficient as a sacrifice for me?" The Prophet replied, "Yes. But it will not be sufficient for anyone else (as a sacrifice), after you."