פירוש על עבודה זרה 75:1

Rashi's commentary: The rabbis decreed [that one must not eat "stewed" or cooked foodstuffs of a non-Jew] because of the concern lest they eat impure things. And more [than this concern] it seems that the reason [for this decree] is because of [the concern] of intermarriages [between Jews and non-Jews]. And thus [in] Chapter 5 of the Mishnah, the great Rabbi Abraham son of Rabbi David said certainly lashes [are given because] the sages forbade a non-Jew from cooking in his house [for a Jew], but if he [i.e. the non-Jew] cooks in the house of a Jew, there is no suspicion of either intermarriage or of eating impure things. But Rabbeinu Tam did not agree with him, that certainly when it comes to a non-Jew cooking, the sages did not divide the general principle [of forbidding food cooked by a non-Jew] between the domain of the Jew and the domain of the non-Jew, because there is always a suspicion lest one is not careful also in the home of the Jew in the same manner that one is careful in the home of the non-Jew