Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.021C.

Ibn ‘Umar related that the Messenger of Allah said, “Differ from the polytheists: let your beards (grow)” and shave your moustache.” (Related by al-Bukhari and Muslim). Abu Hurairah reported that the Messenger of Allah, upon whom be peace, said, “Five things are part of one’s fitra: Shaving the pubic hairs, circumcision, trimming the moustache, removing the hair under the arms and trimming the nails.” (Related by “the group.”) It does not mention which one would specifically fulfil the sunnah. One should make sure that his moustache is not so long that food particles, drink and dirt accumulate in it. Zaid ibn Arqam related that the Prophet, upon whom be peace, said “Whoever does not take (off) some of his moustache is not one of us.” (Related by Ahmad, an-Nasa’i and at-Tirmizhi, who classified it as sahih.) It is preferred to cut the pubic hairs, pluck out the underarm hairs, cut the nails and trim the moustache on a weekly basis, a practice which is most hygienic. If some unnecessary hair is left on the body for a longer period of time, it may disturb the person. One may leave this action for forty days, but no longer. Said Anas, “The time period for us to trim the moustache, cut the nails, pluck out the underarm hairs and cut the pubic hairs was forty nights.” (Related by Ahmad, Abu Dawud and others).