Musannaf Ibn Abi Shayba 13:1879

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Shu’bah, from al-Hakam, from Ibrahim, who said: If a slave testifies and his testimony is rejected, then he is emancipated, he (Ibrahim) said: It is not permissible. And al-Hakam said: It is permissible.