Parashas Metzora · Passuk 15:16
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 43a.
English: “When it goes out from him” — a baal keri does not convey impurity until his impurity exits outward, as it is written “when it goes out from him.”
Exit required
Chazal require that the emission actually leave the body before impurity begins.