From Sahih Bukhari (Hadith 5473):
Narrated Abu Huraira:The Prophet (ﷺ) 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
Reference: Bukhari 5473