Musnad Ahmad 1312:0

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