Parashas Metzora · Passuk 15:02
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 43a (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “When he has a discharge from his flesh” — the zav does not become tamei until his impurity emerges outside, as it is said “when he has a discharge from his flesh” — until his discharge leaves his flesh.
External emergence required
Chazal require that the discharge actually exit the body before the status of zav takes effect.