Musnad Ahmad 1153:1232

'Affan narrated to us, Hammad bin Salamah narrated to us, 'Abdullah bin 'Uthman informed us, from Yusuf bin Mahak, from Hafsah bint 'Abdur-Rahman, from 'Aishah, who said: "The Messenger of Allah (ﷺ) commanded us to sacrifice one sheep for a female child, and two sheep for a male child. And he commanded us regarding the *far'a* (the firstborn of livestock), that for every five sheep, there is one sheep."