Sahih Muslim Book 22, Hadith Number 4831.

Chapter : The proper time for sacrifice.

Al-Bara’ b. ‘Azib reported that Abu Burda slaughtered the animal as a sacrifice before the (‘Id) prayer. Thereupon Allah’s Apostle (may peace be upon him) said: Offer a substitute for it (since it does not absolve you of the responsibility of sacrifice). Thereupon he said: Allah’s Messenger. I have nothing with me but a goat of less than six months. Shu’ba (one of the narrators) said: I think he (al-Bara’ b. ‘Azib also) said: And it is better than a goat of one year. Thereupon Allah’s Messenger (may peace be upon him) said: Make it a substitute for that (and sacrifice it), but it will not suffice for anyone (as a sacrifice) after you.