Musannaf Ibn Abi Shayba 13:2880

Abu Bakr narrated to us, he said: Jarir narrated to us, from Mughira, from al-Harith, regarding a man who says: "I was a slave to so-and-so," or "My mother was a slave to so-and-so," and then that person says: "You are my slaves today." He said: If they have already been freed by manumission and it is known that they are freedmen (mawali), they do not become slaves to those who claim them, unless he brings just witnesses who testify that they are his slaves today.