Musnad Abu Ya'la al-Mawsili 42:233

Zuhayr narrated to us, Waki' narrated to us, Ibn Abi Layla narrated to us, from 'Atiyyah, from Abu Sa'id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The slaughter of the fetus is the slaughter of its mother."