Avodah Zarah 33A

דאביק בה, מהדר הדר אזיל.

he will cleave to it and go to worship idolatry once again.

והתניא: ישראל ההולך לתרפות, בין בהליכה בין בחזרה ⁠— אסור! אמר רב אשי: כי תניא ההיא בישראל משומד, דודאי אזיל.

The Talmud asks: But isn’t it taught in a baraita that with regard to a Jew who goes to a festival of idolatry, whether he is on the way there or returning from it, it is prohibited to engage in business with him? The Talmud answers that Rav Ashi said: When that baraita is taught, it is in reference to an apostate Jew, who will certainly go to the festival, and will certainly not change his mind.

תנו רבנן: גוי ההולך ליריד, בין בהליכה בין בחזרה ⁠— מותר. ישראל ההולך ליריד, בהליכה ⁠— מותר, בחזרה ⁠— אסור.

A baraita states: With regard to a non-Jew who goes to a fair, where goods and idols are bought and sold, whether he is on the way there or returning from it, it is permitted to engage in business with him. With regard to a Jew who goes to a fair, when he is on the way there, it is permitted to engage in business with him, but if he is returning from it, it is prohibited.

מאי שנא ישראל דבחזרה אסור? דאמרי: עבודה זרה זבין, דמי עבודה זרה איכא בהדיה. גוי נמי נימא: עבודה זרה זבין, דמי עבודה זרה איכא בהדיה!

The Talmud asks: What is different about a Jew that it is prohibited to engage in business with him when he is returning? The Talmud answers that this is in accordance with that which the rabbis say: He sold objects of idol worship, and therefore money gained from idol worship is in his possession. Since the money in his possession is assumed to have been gained through objects of idolatry, it is prohibited to engage in business with the Jew. The Talmud challenges: But with regard to a non-Jew, also, let us say: He sold objects of idol worship, and therefore money gained from idol worship is in his possession. Why is it prohibited to do business only with the Jew?