Sahih Muslim
The Book of Sacrifices (Kitab Al-Adahi)
Book Number: 022 / Hadith Number: 4835
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 thern 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).