Jami` at-Tirmidhi 2759
جامع الترمذي#2759SahihNarrated Anas bin Malik:"[The Messenger of Allah (ﷺ)] 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
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih Muslim
In-book reference: 43:29