Musannaf Ibn Abi Shayba 16:76

Abu Bakr narrated to us, he said: Abdullah bin Uthman bin Khuthaym narrated to us, from Yusuf bin Mahak, from Hafsa bint Abd al-Rahman, from Aisha, that she said: The Messenger of Allah, may Allah bless him and grant him peace, commanded us regarding the *fara'* (the firstborn offspring of livestock), that for every five sheep, there is one sheep.