Musannaf Ibn Abi Shayba 13:417

Ibn Numayr narrated to us, from Ishaq ibn Maysara ibn al-Ghusayn, who said: I heard al-Sha'bi—and he wanted to make a Christian swear an oath, so he said: "Swear by Allah." Al-Sha'bi said: "You have abandoned Allah while you are seeing. Take him to the church and make him swear by that which the people of his religion swear by."