If I can recall it would be found in Kedushin (besides others)
סנהדרין פב א
קריינא דאיגרתא איהו ליהוי פרוונקא
The reader of the letter (that contains the orders), let him be the one to carry it out.
The context there is Pinchas telling Moshe, didn't you taught us that if one is having intercourse with Aramine women, is to be killed by zealots? and that's Moshe answer to him.