הלכה על עבודה זרה 39:2

He prohibited us from at all pitying idolaters and from lauding anything particular to them. And that is His saying, "do not show them mercy (techonem)" (Deuteronomy 7:2). And the tradition comes [to teach]: Do not show them favor; to the point that it is forbidden for us to say, "This one has a nice form," when an idolator has a nice form - as is explained in our Gemara (Avodah Zarah 20a). And in the Gemara Yerushalmi (Yerushalmi Avodah Zarah 1:9), it said, "You shall not show them favor, [involves] a negative commandment." (See Parashat Vaetchanan; Mishneh Torah, Foreign Worship and Customs of the Nations 10.)