Asalaamu Alaikum, According to Sheikh Mufti Muhammad ibn Adam:
The great Hadith master, Imam al-Nawawi (Allah have mercy on him) states in his renowned commentary of Sahih Muslim:
The words: A'fu, Awfu. Arkhu, Arju and Waffiru have all been used; the meaning of all of which is to leave the beard as it is." (Sharh Sahih Muslim, 3/151)
In the above narrations, the Messenger of Allah (Allah bless him & give him peace) explicitly commanded the growing of the beard. According to the scholars of the
"science of juristic principles" (usul al-Fiqh), the explicit command (amr) of the Qur'an and Sunnah signifies the obligatory nature of an act (al-Amr li al-Wujub) unless otherwise proven.
(See, for example: al-Nasafi, Kashf al-Asrar Sharh al-Manar, 1/37 for the Hanafi school, al-Baji, Ihkam al-Fusul fi Ahkam al-Usul, p.79 for the Maliki school, al-Shirazi, al-Luma' for the Shafi'i school and Ibn Badran, Nuzhat al-Khatir al-Atir, 2/43 for the Hanbali school).
The Messenger of Allah's (Allah bless him & give him peace) constant practice was also that of keeping a beard:
5) Abu Ma'mar relates that we asked Khabbab (Allah be pleased with him) whether the Messenger of Allah (Allah bless him & give him peace) used to recite the Qur'an in the Zuhr and the Asr prayers. He replied in the affirmative. We said,
"How did you come to know of this?" He replied,
"From the movement of his beard." (Sahih al-Bukhari1/971 & Sunan Abu Dawud, no. 801)
6) Sayyiduna Anas ibn Malik (Allah be pleased with him) narrates that when the Messenger of Allah (Allah bless him & give him peace) performed ablution (wudhu), he would take a handful of water and put it under his jaws and pass it through his beard. He said,
"This is what my Lord ordered me to do." (Sunan Abu Dawud, no. 145, Sunan al-Bayhaqi, 1.54 and authenticated by al-Hakim)
7) Sayyiduna Jabir ibn Abd Allah (Allah be pleased with him) narrates
that the Messenger of Allah (Allah bless him & give him peace).......had a thick beard...." (Sahih Muslim, no. 1823)
The four Sunni schools of Islamic law (madhhabs) also quite clearly mention the impermissibility of shaving the beard:
The Hanafi School
The great Hanafi jurist, Imam al-Haskafi (Allah have mercy on him) states in his Durr al-Mukhtar:
"It is unlawful for one to trim his beard....And as for shortening it when it is less than a fistful, as some North Africans and effeminate men do, this is something no one (i.e. of the Hanafi scholars) has said is permitted." (See: Radd al-Muhtar ala al-Durr al-Mukhtar, 2/113, Kitab al-Sawm)
This position is also confirmed in other works, such as al-Bahr al-Ra'iq, Fath al-Qadir, al-Fatawa al-Hindiyya, Bada'i al-Sana'i and al-Ikhtiyar.
The Maliki School
Imam al-Dasuqi (Allah have mercy on him) states:
"It is unlawful (haram) for a man to shave off his beard, and the perpetrator will be disciplined." (Hashiyat al-Dasuqi ala Sharh al-Kabir, 1/90, Kitab al-Taharah)
The Shafi'i school
It is stated in Sharh al-Ubab:
"The two shaykhs of the school (i.e. Nawawi & Rafi'i) state that it is Makruh (disliked) to shave the beard, but Ibn al-Rif'a objected to this stating that Imam Shafi'i (Allah have mercy on him) himself categorically mentioned in his book "al-Umm" that shaving the beard is unlawful (haram). Imam al-Azra'i said that the correct position of the school is that, to shave the beard without a valid medical reason is unlawful." (Sharh al-Ubab & Hashiya Ibn Qasim al-Abbadi, 9/376)
Imam Ibn Qasim al-Abbadi also stated in his Hashiya:
"It is said that, shaving of the beard is unlawful." (See: Hashiya Ibn Qasim al-Abbadi ala Tuhfat al-Muhtaj, 2/468, Kitab al-Salat)
The Hanbali School
Imam al-Bahuti (Allah have mercy on him) states:
"(From the acts of nature (fitrah) is to lengthen the beard in a way that it should not be trimmed at all. It is stated in the school that, this is as long as it does not become abnormally long in a way that it causes abhorrence. And it is unlawful (haram) to shave it off." (Kashaf al-Qina', 1/75).
This is also confirmed in the other major Hanbali works, such as: al-Insaf, 1/121 by al-Mawardi, Sharh Muntaha al-Iradat, 1/85 by al-Bahuti and al-Rawdh al-Murbi', 27-28 also by al-Bahuti.
The above statements of the Messenger of Allah (Allah bless him & give him peace) and the statements of the jurist (fuqaha) go a long way in proving that to shave the beard (or trimming it to a very thin line which in effect is shaving) is unlawful (haram). A Muslim (and especially one who classes himself to be practicing) can never look with favour towards the act of shaving the beard.
Read in more detail here:
http://www.daruliftaa.com/question.asp?txt_QuestionID=q-18192582
And Allah knows best in all matters