From Sahih Muslim (Hadith 3810):
Ibn 'Umar (Allah be pleased with them) reported that the people of pre-Islamic days used to sell the meat of the slaughtered camel up to habal al-habala. And habal al-habala implies that a she-camel should give birth and then the (born one should grow young) and become pregnant. Allah's Messenger (ﷺ) forbade them that (this transaction)
Reference: Muslim 3810