BabylonianTalmud, Avodah Zara 28a

Rabbi Yochanan suffered from a tooth ailment, so he went to an idolatrous noblewoman who was known as a healer. She prepared a salve for him on Thursday and Friday. He said to her: How shall I do it tomorrow, on Shabbat? She replied: You won’t need it on Shabbat. R. Yochanan asked: And if I do need treatment, what should I do? She replied: Swear to me that you will not reveal the remedy to anyone. R. Yochanan swore to her “To the God of Israel, I will not reveal.” She revealed the secret remedy to him, and the next day he went out and taught the remedy in his lecture. The Gemara expresses surprise that R. Yochanan revealed the remedy, since he had sworn not to. But he swore that he would not!! Rather, he swore that he would not reveal the secret to the God of Israel, but to the People of the God of Israel, he would reveal it! But isn’t it a desecration of God’s name to deceive her in that way? He revealed his intentions to her from the beginning. Thus we see that a tooth ailment is like an internal infection. R. Nachman bar Yitzhak says that tsafdina is different, since it begins in the mouth and ends in the intestines. [Soncino translation]