Jami` at-Tirmidhi Book 43, Hadith 28
from the Prophet (ﷺ), that he (ﷺ) fixed the time as every forty nights for them to trimming the fingernails, taking from the mustache, and shaving the pubic hairs.
that the Prophet (ﷺ) said: "Ten are from the Fitrah: Paring the mustache, leaving the beard to grow, Siwak, cleaning the nose with water, paring the fingernails, washing the knuckles, plucking the underarm hair, shaving the pubic hairs, and Intiqas with water."
" fixed the time for us paring the mustache, trimming the fingernails, shaving the pubic hairs and plucking the underarm hairs - that we not leave it for more than forty days."