Musnad Ahmad 30:633

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no *Fara'* and no *'Atirah*." The *Fara'* was the first offspring that would be born to them, so they would slaughter it.