Prophet (peace be upon him). He said, "From the acts of nature are five: circumcision (obligation for men, but not for women), removing pubic hairs, trimming the mustache, cutting the nails and plucking the hair from under the armpits."1 [Recorded by al-Bukhari and Muslim].
 

It is not allowed to leave them for more than forty nights. This is based on the Hadith of Anas who said, "The Messenger of Allah set a time limit for us for trimming the mustache, trimming nails, removing armpit hairs and removing pubic hairs. They cannot be left for more than forty nights."2 [Recorded by Muslim]. Letting them grow long resembles animals and some of the disbelievers.