Musar על במדבר 16:21

For this reason we must be wary when the other nations make feasts not to derive any benefit from them, as mentioned above. The only exception is when the non-Jew sends animals, poultry, fish and the like to the home of the Jew. Then it is permissible. But it is forbidden for a Jew to eat in the home of the non-Jew, even if he consumes only kosher food and drinks only kosher wine. Whoever is lax about this commits a grievous sin and incurs severe punishment. Moreover, the Shechinah cries profusely over such people. The proof is that it was on account of this sin that Israel was considered deserving of death in the days of Haman and Achashveirosh. That is why Mordechai alone refused to benefit from the feast of that evildoer in order that in his merit all of Israel would be saved. But even so, the attribute of judgment wished to strike against all the Jews of that generation. For just as Hashem said to Moshe, “I will consume them in a moment” (Bamidbar 16:21), and “I will make you into a great nation [in their stead]” (Shemos 33:10), so too the attribute of judgment wished to multiply Mordechai’s seed like the sand of the sea [in Israel’s stead]. But Mordechai nullified the decree with his prayer just as Moshe Rabbeinu had nullified the decree against the Jews of his day with his prayer.