Musnad Ahmad 28:261

Abdullah ibn al-Harith al-Makki narrated to us, Abdullah ibn Amir al-Aslami narrated to me, from Amr ibn Shu'ayb, from his father, from his grandfather, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed the 'aqiqah for the boy with two sheep, and for the girl with one sheep."