All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
There is no harm for the person in a state of Janaabah (ritual impurity) to trim his nails or hair; for, in principle, it is permissible and there is no authentic evidence proving the contrary. Whoever claims that act to be impermissible, should be asked to support his claim with evidence.
Shaykhul-Islam Ibn Taymiyyah was asked, as cited in Majmoo‘ Al-Fataawa: “If a man cuts his nails or moustache or combs his hair when he is Junub, is there any blame on him for that? Some people argue that if the Junub cuts his hair or nails, his cut parts will come back to him in the Hereafter and he will be raised on the Day of Resurrection with a share of Janaabah commensurate with whatever was removed, and for every hair he will have a share of this state of Janaabah; is this true or not?”
Shaykhul-Islam Ibn Taymiyyah replied,
“It is proven in the narrations that were reported on the authority of Abu Hurayrah and Huthayfah that when the Prophet, sallallaahu ‘alayhi wa sallam, was asked about the Junub he said: “The believer does not become impure.”
The same narration was reported in Saheeh Al-Haakim with the following addition: “whether alive or dead.”
Furthermore, I do not know of any evidence in the Islamic Sharee‘ah proving that it is disliked for the Junub to trim his hair or nails. In fact, the Prophet, sallallaahu ‘alayhi wa sallam, told the man who embraced Islam: “Rid yourself of the hair of kufr and get circumcised.”
Hence, the Prophet, sallallaahu ‘alayhi wa sallam, ordered the new Muslim to perform Ghusl (ritual bath), yet he did not order him to delay circumcision and shaving his hair until after performing Ghusl. Indeed, the general meaning of his words implies that both courses of action are permissible. Similarly, the menstruating woman is enjoined to comb her hair when performing Ghusl even though combing the hair makes some hair fall out."
Allaah Knows best.