Sahih Bukhari Volume 007, Book 068, Hadith Number 453.
Narrated By Al-Bara : The Prophet said (on the day of Idal-Adha), "The first thing we will do on this day of ours, is to offer the ('Id) prayer and then return to slaughter the sacrifice. Whoever does so, he acted according to our Sunna (tradition), and whoever slaughtered (the sacrifice) before the prayer, what he offered was just meat he presented to his family, and that will not be considered as Nusak (sacrifice)." (On hearing that) Abu Burda bin Niyar got up, for he had slaughtered the sacrifice before the prayer, and said, "I have got a six month old ram." The Prophet said, 'Slaughter it (as a sacrifice) but it will not be sufficient for any-one else (as a sacrifice after you). Al-Bara' added: The Prophet said, "Whoever slaughtered (the sacrifice) after the prayer, he slaughtered it at the right time and followed the tradition of the Muslims."