From Sahih Bukhari (Hadith 5500):
Narrated Jundub bin Sufyan Al-Bajali:Once during the lifetime of Allah's Messenger (ﷺ) we offered some animals as sacrifices. Some people slaughtered their sacrifices before the (Id) prayer, so when the Prophet (ﷺ) finished his prayer, he saw that they had slaughtered their sacrifices before the prayer. He said, "Whoever has slaughtered (his sacrifice) before the prayer, should slaughter (another sacrifice) in lieu of it; and whoever has not yet slaughtered it till we have prayed; should slaughter (it) by mentioning Allah's Name
Reference: Bukhari 5500