פירוש על קידושין 70:11

The Talmud now asks why not say that indeed “sons of Aaron” refers only to the first commandments in Leviticus 21 and that the comparison of the verses (the gezerah shavah) is there to teach what instruments are included in the prohibition to shave—only a razor and not scissors, pincers or some sort of remover.
This returns us to our original question—how do we know that women are exempt from this prohibition?