Musannaf Ibn Abi Shayba 13:3097

Abu Bakr narrated to us, he said: Zayd ibn al-Hubab narrated to us, from Hammam, from Qatadah, from al-Hasan, regarding a man who says to his slave, "If you do such and such, then you are free," or to his wife, "Then you are divorced." He said: If there was between them a sale or a divorce, it does not take effect.