Musnad Ahmad 1153:2115

'Abd al-Samad narrated to us, he said: Hammad narrated to us, 'Abdullah ibn 'Uthman narrated to us, from Yusuf ibn Mahak, from Hafsah bint 'Abd al-Rahman, from 'Aishah, that she said: "The Messenger of Allah (peace and blessings of Allah be upon him) commanded us regarding the *fara'* (the firstborn offspring of livestock) that for every five sheep there is one sheep. And he commanded us to perform the *'aqiqah* (sacrifice) for a female child with one sheep, and for a male child with two sheep."