Musannaf Ibn Abi Shayba 19:622

Abu Bakr narrated to us, he said: Abd al-A’la narrated to us, from Ma’mar, from al-Zuhri, regarding the Jew and the Christian: If he had been married (muhsan) while in his state of polytheism, then he embraced Islam, and then committed an act of indecency (adultery) before he had been married (muhsan) in Islam, he said: He is to be stoned.