Musnad Ahmad 31:270

Yahya ibn Zakariya ibn Abi Za'idah narrated to us, Mujalid narrated to us, from Abu al-Wadak, from Abu Sa'id al-Khudri, who said: We asked the Messenger of Allah, peace and blessings of Allah be upon him, about the fetus that is in the belly of a she-camel, a cow, or a sheep. He said: "Eat it if you wish, for its slaughter is the slaughter of its mother."