It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that in the time of ignorance, people used to sell the meat of a camel in exchange for the unborn offspring of a pregnant animal, and by the unborn offspring of a pregnant animal is meant "the one who is still in the mother's womb." The Prophet Muhammad (peace and blessings be upon him) forbade this.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4640