Sahih Muslim Book 22, Hadith Number 4827.

Chapter : The proper time for sacrifice.

Al-Bara’ b. ‘Azib reported Allah’s Messenger (may peace be upon him) having said: The first (act) with which we started our day (the day of ‘Id-ul-Adha) was that we offered prayer. We then returned and sacrificed the animals and he who did that in fact adhered to our Sunnah (practice). And he who slaughtered the (animal on that day before the ‘Id prayer), for him (the slaughtering of animal was directed to the acquiring of) meat for his family, and there is nothing of the sort of sacrifice in it. It was Abu Burda b. Niyar who had slaughtered (the animal before the ‘Id prayer). He said: I have a small lamb, of less than one year, but better than that of more than a year. Thereupon Allah’s Messenger (may peace be upon him) said: Sacrifice it, but it will not suffice (as a sacrifice) for anyone after you.