Musnad Ahmad 1268:5

'Abd al-Razzaq narrated to us, he said: Ibn Jurayj informed us, he said: 'Ubayd Allah ibn Abi Yazid informed me, from Siba' ibn Thabit, that Muhammad ibn Thabit ibn Siba' informed him, that Umm Kurz informed him, that she asked the Messenger of Allah, may Allah bless him and grant him peace, about the 'Aqiqah (sacrifice for a newborn). He said: "For the boy, two sheep, and for the girl, one. And it does not harm you whether they are male or female."