Musnad Ahmad 1153:517

'Affan narrated to us, he said: Wuhayb narrated to us, 'Abdullah ibn 'Uthman narrated to us, from Yusuf ibn Mahak, from Hafsah bint 'Abd al-Rahman ibn Abi Bakr, from her paternal aunt 'Aishah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded us regarding the *far'ah* (the firstborn offspring) of sheep: one out of every five."