Musnad Abu Ya'la al-Mawsili 150:5

Suwayd ibn Sa'id narrated to us, Yahya ibn Sulaym narrated to us, from Ibn Khuthaym, from Yusuf ibn Mahak, that Hafsah bint 'Abd al-Rahman narrated to him, from 'Aishah, that she heard the Messenger of Allah (may Allah bless him and grant him peace) commanding the *far'ah* (the firstborn offspring) of sheep: from every five, one.