Parashas Metzora · Passuk 15:08
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 56a (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “And if he spits” — the sputum, mucus, and spit of a zav convey impurity when moist, and do not convey impurity when dry; what is the reason? As it is written “and if the zav spits” — in the manner of spit [at the time of spitting].
Moist only
Chazal restrict the impurity of the zav’s spit-related fluids to the moist state.