Oaths and Vows

Bukhari :: Book 8 :: Volume 78 :: Hadith 666

Narrated 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."