Musnad Ahmad 1304:22

Haytham ibn Kharijah narrated to us, he said: Isma'il ibn 'Ayyash narrated to me, from Thabit ibn al-'Ajlan, from Mujahid, from Asma' bint Yazid, from the Prophet, peace and blessings of Allah be upon him, who said: "The 'Aqiqah for the boy is two equivalent sheep, and for the girl is one sheep."