Musnad Ahmad 1153:19

Bishr ibn al-Mufaddal narrated to us, from Abdullah ibn Uthman, from Yusuf ibn Mahak, who said: We entered upon Hafsa bint Abd al-Rahman, and she informed us that Aisha informed her that the Messenger of Allah, peace and blessings of Allah be upon him, said: "For the boy, two equivalent sheep, and for the girl, one sheep."