Sahih Bukhari Volume 007, Book 068, Hadith Number 457.
Narrated By Anas bin Malik : The Prophet said on the day of Nahr, "Whoever has slaughtered his sacrifice before the prayer, should repeat it (slaughter another sacrifice)." A man got up and said, "O Allah's Apostle! This is a day on which meat is desired." He then mentioned his neighbors saying, "I have a six month old ram which is to me better than the meat of two sheep." The Prophet allowed him to slaughter it as a sacrifice, but I do not know whether this permission was valid for other than that man or not. The Prophet then went towards two rams and slaughtered them, and then the people went towards some sheep and distributed them among themselves.