In Parshat Ki Teitzei there's the mitzvah that Bnei Ammon cannot marry into the Jewish people. Same is true for Bnei Moav. The verse says it's because they didn't greet the Jews with bread and water.
Rashi (23:9) says that we learn from here that causing others to sin is worse than killing them, since you kill them in this world and in the next world. This is because Bnei Moav hired Bilaam and the Benot Midian to seduce the Jews.
However, I don't recall the Torah mentioning anything about Bnei Ammon causing the Jews to sin. What is Rashi (quoting Sifre) referring to?