Praise be to Allaah.
Cutting the nails is part of the fitrah, because the Prophet (peace and blessings of Allaah be upon him) said: “The fitrah is five things: circumcision, removing the pubic hair, trimming the moustache, cutting the nails, and plucking the armpit hairs.” (Reported by al-Bukhaari and Muslim). According to another hadeeth, ten things are part of the fitrah, including cutting the nails. Anas (may Allaah be pleased with him) said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) set a time limit for us to cut our moustaches, cut our nails, pluck our armpit hairs and shave our pubic hairs: we were not to leave it any longer than forty days.” (Reported by Ahmad, Muslim and al-Nisaa’i; this version narrated by Ahmad). Whoever does not cut his nails is going against the fitrah. The reason behind this is cleanliness and hygiene, because dirt can gather under the nails. It also serves the purpose of making us different from those among the kuffaar who allow their nails to grow long, and from animals that have claws and talons. (Fataawa al-Lajnah al-Daa’imah, 5/173).
Most women nowadays have fallen into the trap of resembling savage animals with their long claw-like nails, which they then paint with nail polish. This ugly appearance looks utterly repulsive to any intelligent person whose fitrah is sound. Another bad habit that many people have is to leave one nail long. All of this is clearly contrary to the ways of the fitrah. We ask Allaah to keep us safe and sound. Allaah is the Guide to the Straight Path.
Sheikh Muhammed Salih Al-Munajjid