Musannaf Ibn Abi Shayba 13:627

Abu Bakr narrated to us, he said: Hushaym narrated to us, from Mughira, from Ibrahim, who said: Whoever sells a pregnant animal or emancipates it and makes an exception for what is in its womb, he said: His exception is valid regarding that whose creation has become apparent. But if its creation has not become apparent, then he has nothing.