Musannaf Ibn Abi Shayba 13:631

Abu Bakr narrated to us, he said: Yahya ibn Yaman narrated to us, from Sufyan, from Ibn Jurayj, from ‘Ata’. And from Sufyan, from Jabir, from al-Sha‘bi, and from Sufyan, from Mansur, from Ibrahim: If he emancipates her and makes an exception for what is in her womb, then the exception is valid for him.