Sahih Bukhari Volume 002, Book 015, Hadith Number 074.
Narated By Anas : The Prophet said, "Whoever slaughtered (his sacrifice) before the 'Id prayer, should slaughter again." A man stood up and said, "This is the day on which one has desire for meat," and he mentioned something about his neighbors. It seemed that the Prophet I believed him. Then the same man added, "I have a young she-goat which is dearer to me than the meat of two sheep." The Prophet permitted him to slaughter it as a sacrifice. I do not know whether that permission was valid only for him or for others as well.