Musannaf Ibn Abi Shayba 13:1878

Abu Bakr narrated to us, he said: Mu'adh ibn Mu'adh narrated to us, from Ash'ath, from al-Hasan, that he used to say regarding the slave and the Dhimmi: If they both testified and their testimony was rejected, then this one was emancipated and this one accepted Islam, their testimony becomes permissible.