Rashi on Numbers 25:7/רש"י על במדבר כ״ה:ז׳

1וירא פנחס AND PHINEAS SAW — He saw what was being done and he was thereby reminded of the law on this subject (Sanhedrin 82a). He said to Moses, “I have received a tradition from you: he who has intercourse with an Aramean (heathen) woman, zealous people may attack him”. He replied to him: “Let him who reads the letter be the agent for executing it”; — straightway, ויקח רמח בידו HE TOOK A JAVELIN IN HIS HAND, etc. (Sanhedrin 82a).