Sahih Bukhari Volume 7, Book 66, Hadith Number 382.

Narrated By Abu Huraira : The Prophet said, “Neither Fara’ nor ‘Atira (is permissible):” Al-Fara’ nor ‘Atira (is permissible):” Al-Fara’ was the first offspring (of camels or sheep) which the pagans used to offer (as a sacrifice) to their idols. And Al-‘Atira was (a sheep which was to be slaughtered) during the month of Rajab.