(A man buys a she-camel and stipulates that its payment will be made when the female bears young, and the female becomes pregnant and then gives birth to a female.)
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