Muhammad bin Abi Bakr (may Allah have mercy on him) wrote to Sayyiduna Ali (may Allah be pleased with him) that a Muslim had committed adultery with a Christian woman. He replied to him: Establish the prescribed punishment on the Muslim, and return the woman to the people of her religion.
Imam al-Shafi'i (may Allah have mercy on him) said: If this is established, then it shows that he has the choice to judge or not to judge.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحدود / 17121