Narrated Al-Bara':
The Prophet delivered the Khutba on the day of Nahr ('Id-ul-Adha) and said, "The first thing we should do on this day of ours is to pray and then return and slaughter (our sacrifices). So anyone who does so he acted according to our Sunna; and whoever slaughtered before the prayer then it was just meat that he offered to his family and would not be considered as a sacrifice in any way. My uncle Abu Burda bin Niyyar got up and said, "O, Allah's Apostle! I slaughtered the sacrifice before the prayer but I have a young she-goat which is better than an older sheep." The Prophet said, "Slaughter it in lieu of the first and such a goat will not be considered as a sacrifice for anybody else after you."