Sahih Muslim Book 022, Hadith Number 4823.
Chapter : The proper time for sacrifice.
Al-Bara' reported: My maternal uncle Abu Burda sacrificed his animal before ('Id) prayer. Thereupon Allah's Messenger (may peace be upon him) said: That is a goat (slaughtered for the sake of) flesh (and not as a sacrifice on the day of Adha). He said: I have a lamb of six months. Thereupon he said: Offer it as a sacrifice, but it will not justify for anyone except you, and then said: He who sacrificed (the animal) before ('Id) prayer, he in fact slaughtered it for his own self, and he who slaughtered after prayer, his ritual of sacrifice became complete and he in fact observed the religious practice of the Muslims.