Mesirah (or mesira, lit. 'to hand over') is the action in which one Jew reports the conduct of another Jew to a non-rabbinic authority in a manner and under the circumstances forbidden by rabbinic law.

https://en.wikipedia.org/wiki/Mesirah

https://www.quora.com/What-parts-of-the-Talmud-are-anti-Gentile