Abu Dawud
Sacrifice
Book Number: 009 / Hadith Number: 2828
Narated By Umm Kurz al-Ka'biyyah : I heard the Apostle of Allah (pbuh) say: Two resembling sheep are to be sacrificed for a boy and one for a girl.
Abu Dawud said: I heard Ahmad (ibn Hanbal) say: The Arabic word mukafi'atani means equal (in age) or resembling each other.