Sahih Bukhari Volume 7, Book 71, Hadith 5475

حَدَّثَنَا عَبْدَانُ، حَدَّثَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا مَعْمَرٌ، أَخْبَرَنَا الزُّهْرِيُّ، عَنِ ابْنِ الْمُسَيَّبِ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏"‏ لاَ فَرَعَ وَلاَ عَتِيرَةَ ‏"‏‏.‏ وَالْفَرَعُ أَوَّلُ النِّتَاجِ، كَانُوا يَذْبَحُونَهُ لَطِوَاغِيتِهِمْ، وَالْعَتِيرَةُ فِي رَجَبٍ‏.‏

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.