Musnad Ahmad 31:505

Yahya ibn Sa'id narrated to us, from Mujalid, Abu al-Waddak narrated to us, from Abu Sa'id, who said: We asked the Messenger of Allah, peace and blessings of Allah be upon him, about the fetus of a she-camel and a cow. He said: "If you wish, eat it, for its slaughter is the slaughter of its mother."