Hilchot Issurei Bi’ah, Chapter 12, Halacha 10 (Rambam, trans. Eliyahu Touger):
[10] If, by contrast, a Jewish male enters into relations with a gentile woman, when he does so intentionally, she should be executed. She is executed because she caused a Jew to be involved in an unseemly transgression, as is the law with regard to an animal. This applies regardless of whether the gentile woman was a minor of three years of age, or an adult, whether she was single or married. And it applies even if the Jew was a minor of nine years old, she is executed.
This punishment is explicitly mentioned in the Torah, as Numbers 31:16-17 states: “Behold they were involved with the children of Israel according to the advice of Balaam ... Execute any woman fit to know a man through lying with a male.”
Hebrew: אֲבָל יִשְׂרָאֵל שֶׁבָּא עַל הַגּוֹיָה - בֵּין קְטַנָּה בַּת שָׁלוֹשׁ שָׁנִים וְיוֹם אֶחָד בֵּין גְּדוֹלָה, בֵּין פְּנוּיָה בֵּין אֵשֶׁת אִישׁ, וְאַפִלּוּ הָיָה קָטָן בֶּן תֵּשַׁע שָׁנִים וְיוֹם אֶחָד - כֵּיוָן שֶׁבָּא עַל הַגּוֹיָה בְּזָדוֹן, הֲרֵי זוֹ נֶהְרֶגֶת... (full text matches standard editions).