Sunan Ibn Majah
295
Sahih+2

It was narrated that Anas bin Malik said:"We were given a time limit with regard to trimming the mustache, shaving the pubic hairs, plucking the armpit hairs and clipping the nails. We were not to leave that for more than forty days

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi