Musannaf Ibn Abi Shayba 35:102

Hafs and Abd al-Rahim ibn Sulayman narrated to us, from al-Mujalid, from Abu al-Wadak Jabr ibn Nawf, 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, if it has grown hair." And it was mentioned that Abu Hanifa said: "Its slaughter is not the slaughter of its mother."