Sahih Muslim Book 22, Hadith Number 4835.

Chapter : The proper time for sacrifice.

Anas b. Malik reported: Allah’s Messenger (may peace be upon him) addressed us on the day of ‘Id al-Adha. He smelt the odour of flesh and he prohibited them from slaughtering (the animals before the ‘Id prayer), saying: He who slaughtered the animals (before the ‘Id prayer) should do that again (as it is not valid as a sacrifice).