I was with Allah's Messenger (ﷺ) on the day of 'Id al-Adha. While he had not returned after having offered (the Id prayer) and finished it, he saw the flesh of the sacrificial animals which had been slaughtered before he had completed the prayer. Thereupon he (the Holy Prophet) said: One who slaughtered his sacrificial animal before his prayer or our prayer ('Id), he should slaughter another one in its stead, and he who did not slaughter, he should slaughter by reciting the name of Allah.

My maternal uncle Abu Burda sacrificed his animal before ('Id) prayer. Thereupon Allah's Messenger (ﷺ) 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.