Muwatta Imam Malik (Riwayat Ibn al-Qasim)

And with this same chain (it is narrated from Sayyiduna Ibn Umar, may Allah be pleased with them both) that the Messenger of Allah, peace and blessings be upon him, forbade the sale of the unborn offspring in the womb of an animal. And this was a transaction that the people of ignorance used to make with each other. A man would make a deal for a she-camel that when the she-camel gives birth to a female, then whatever camel is born from her, that will be mine.