Sahih Bukhari
The Two Festivals (Eids)
Volume: 002 / Book Number: 015 / Hadith Number: 101
Narrated Jundab:
On the day of Nahr the Prophet offered the prayer and delivered the Khutba and then slaughtered the sacrifice and said, "Anybody who slaughtered (his sacrifice) before the prayer should slaughter another animal in lieu of it, and the one who has not yet slaughtered should slaughter the sacrifice mentioning Allah's name on it."