Sayyiduna Abu Sa'id (may Allah be pleased with him) says: We said: O Messenger of Allah (peace and blessings be upon him)! We slaughter a she-camel, a cow or a sheep that has a fetus in its womb; shall we eat that fetus or throw it away? He (peace and blessings be upon him) said: "Eat it if you wish, for the slaughter of its mother counts as the slaughter of the fetus."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19490