Narrated 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." (JA)
English reference (JA): Vol. 5, Book 41, Hadith 2759 (JA)
Arabic reference (JA): Book 43, Hadith 2984 (JA)