Parashas Eikev · Passuk 7:14
You shall be blessed above all other peoples: there shall be no sterile male or female among you or among your livestock.
Torah Temimah points to Yerushalmi Berakhot 8:8.
TT lemma: מכל העמים (unit 1 of 3).
Sefaria — Yerushalmi Berakhot 8:8
English: R’ Tanchum said: If a non-Jew blesses you, answer Amen after him, as it is written, “You shall be blessed by all the peoples.”
Custom (halakhah of answering Amen)
Amen is answered even to a non-Jew’s blessing, based on this verse.