Musannaf Ibn Abi Shayba 13:1876

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ma’mar, from al-Zuhri and Qatadah, they both said: The People of the Scripture, the slave, and the child, if they possess testimony, and the People of the Scripture embrace Islam, and the slave is emancipated, and the child reaches maturity, then their testimony is permissible, unless it was rejected while they were in that [previous] state.