Musnad Ahmad 1221:5

'Affan narrated to us, he said: Hammad bin Zayd narrated to us, he said: 'Ubaydullah bin Abi Yazid narrated to me, he said: Siba' bin Thabit narrated to me, from Umm Kurz, that the Messenger of Allah (peace and blessings of Allah be upon him) said regarding the 'Aqiqah: "For the boy, two equivalent sheep, and for the girl, one sheep."