Parashas Metzora · Passuk 14:16
Chabad.org — English translation + Rashi
Torah Temima points to Menachos 10a (verified on the daf).
E-Daf.com — Tsurras Hadaf (size=3)
HebrewBooks / Shas PDF of the daf
English: “His right finger” — Rabbah bar bar Chanah said in the name of R’ Shimon ben Lakish: Wherever the Torah mentions “finger” and “kehunah,” it refers only to the right hand. Whence do we know this? From the metzora, as it is written “and the kohen shall dip his right finger.”
General rule from the metzora
Chazal derive the general principle that any priestly service requiring a finger must be performed with the right hand, learning it from the explicit wording here.