Sahih Muslim Book 22, Hadith Number 4830.

Chapter : The proper time for sacrifice.

Al-Bara’ b. ‘Azib reported: Allah’s Messenger (may peace be upon him) addressed us on the day of Nahr and said: None should sacrifice the animal unless he has completed the (‘Id) prayer. A person said: I have a milch goat of less than one year, better than two fat goats. Thereupon he said: Sacrifice it, and no goat of less than a year of age will be accepted as sacrifice after you.