Sahih Bukhari Volume 008, Book 078, Hadith Number 666.

Narated By Jundub : I witnessed the Prophet offering the 'Id prayer (and after finishing it) he delivered a sermon and said, "Whoever has slaughtered his sacrifice (before the prayer) should make up for it (i.e. slaughter another animal) and whoever has not slaughtered his sacrifice yet, should slaughter it by mentioning Allah's Name over it."