Allah has chosen certain practices for His Prophets; Muslims are to follow these as well, they are known as Sunan al-Fitrah, or the Muslim's cleanliness obligations. Basically they comprise natural hygiene. They are:
1. Trimming the moustache. The Prophet (peace and blessing be upon him) said:
'Differ from the polytheists, grow the beards and trim the moustache.' (Bukhari & Muslim)
2. Growing beards. This is incumbent upon every male Muslim. The Prophet (peace and blessing be upon him) said:
'Grow the beards and trim the moustache.' (Bukhari).
Allah warns those who disobey His Messenger, saying:
(Let those who disobey his command beware lest they are subjected to a Fitnah or a painful torment.) (24:63).
3. The use of Siwak (tooth-stick). The Prophet (peace and blessing be upon him) said:
'Had it not been for the fact that I would overburden my Ummah (nation) I would have ordered them to use the Siwak, as I ordered them to perform Wudhu.' (Silsilah as-Saheehah)
4. Sniffing water, which is a procedural step of Wudhu.
5. Clipping fingernails and toenails.
6. Washing the knuckles.
7 and 8. Removing hair from the armpits and pubic area (by plucking, shaving or cutting).
9. Istinja, which is the cleaning of the two exits with water.
10. The narrator of the Hadeeth said: 'I forget what the tenth
was, unless it is rinsing the mouth during Wudhu.'
Abu Hurairah (may Allah be pleased with him) narrated that the Prophet (peace and blessing be upon him) said:
Five acts are from the Fitrah:
1. Shaving off pubic hair.
3. Trimming the moustache.
4. Clipping fingernails and toenails.
5. Plucking or shaving off armpit hair.
Question: I have noticed that some people in the Masjid let their nails grow long and they are filled with dirt. Is this befitting for a Muslim? Is their Wudhu valid? Is there a specified time limit for cutting the nails, or for doing other things that are Sunan al-Fitrah (specific Sunnah connected to the natural disposition of man)?
Answer: Praise be to Allah. The nails must be cut at least once every forty days, because the Messenger of Allah (peace and blessing be upon him) specified a time limit within which people should cut their nails, shave their pubic hair, pluck their armpit hair and trim their moustaches, and not leave it for more than forty days.
This is affirmed authentically. Anas (may Allah be pleased with him), who was the servant of the Messenger of Allah (peace and blessing be upon him) said: "He set a time for us to trim our moustaches, cut our nails, pluck our armpit hair and shave our pubic hair; we were not to leave that for more than forty days." (Muslim). It was also narrated by Imam Ahmad (11823) and by alNasaa'I (14) with the wording:
"The Messenger of Allah (peace and blessing be upon him) set a time limit for us, we were not to neglect our nails and moustaches, and shaving the pubic hair and plucking the armpit hair beyond forty days."
Both men and women must pay attention to this matter and not leave the nails, moustache, pubic hair or armpit hair for more than forty days.
Wudhu is valid and is not affected by whatever dirt is under the fingernails, because it is a small amount and may be looked over. (Bin Baz).