Rashi on Exodus 34:15/רש"י על שמות ל״ד:ט״ו

1ואכלת מזבחו AND THOU EAT OF HIS SACRIFICE — You might perhaps think that there is no punishable offence in eating of it, this is not so, but I will account it unto you as though you consent to his idolatrous worship because through this (through partaking of his meals) you will be induced to take your daughters unto your sons (cf. Avodah Zarah 8a).