Musnad Ahmad 1221:0

Narrated Umm Kurz al-Ka‘biyyah: I heard the Messenger of Allah (ﷺ) say: "For the boy, there are two sheep that are equivalent, and for the girl, there is one sheep." I asked him about the 'aqiqah (sacrifice for a newborn), and he said: "For the boy, there are two sheep, and for the girl, there is one sheep. It does not harm you whether they are male or female."