Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) "forbade the sale of the offspring of the pregnant one (habal al-habalah)." This was a sale that the people of the Jahiliyyah used to transact among themselves: they would buy a camel (with payment due) when the she-camel gave birth and then the female that was in her belly gave birth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10860