Narrated Abu Huraira:
The Prophet said, “Neither Fara’ nor ‘Atira) is permissible).” Al-Fara’ was the first offspring (they got of camels or sheep) which they (pagans) used to offer (as a sacrifice) to their idols. ‘Atira was (a sheep which used to be slaughtered) during the month of Rajab.
Sahih Bukhari, Volume 7, Book66, Number 383