From Sahih Muslim (Hadith 5081):
Anas b. Malik reported:Allah's Messenger (ﷺ) 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)
Reference: Muslim 5081