Musnad Abu Ya'la al-Mawsili 150:144

Narrated to us 'Abd al-A'la, narrated to us Muslim ibn Khalid, narrated to me Ibn Khuthaym, from Yusuf ibn Mahak, from Hafsah bint 'Abd al-Rahman ibn Abi Bakr, from 'Aishah, that she said: The Messenger of Allah (may Allah bless him and grant him peace) commanded the 'Aqiqah for the boy to be two equivalent sheep, and for the girl, one sheep.