Praise be to Allah.
It is waajib (obligatory) to let the beard grow, and shaving it is haraam (forbidden). Imaam Ahmad, al-Bukhaari, Muslim and others narrated from Ibn ‘Umar (may Allaah be pleased with him and his father) that the Prophet (peace and blessings of Allaah be upon him) said: “Be different from the mushrikeen: let your beards grow and trim your moutsaches.” Ahmad and Muslim reported from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Cut your moustaches and let your beards grow: be different from the Magians.” Persisting in shaving one’s beard is a major sin (kabeerah); the one who does this should be advised against it, and his action should be denounced, especially if he is in a position of religious leadership.
The idea of being different from the Magians and other mushrikeen does not mean being different from them in every single thing, particularly if they happen to be doing something righteous or good in accordance with the fitrah (natural inclinations of man). What is meant is that we should be different from them in matters where they have gone astray and deviated from the truth and the dictates of the fitrah, and gone against the way of the Prophets and Messengers by shaving their beards. We should differ from them in this case by letting our beards grow and cutting our moustaches, following the guidance of the Prophets and Messengers, and in accordance with the fitrah. It is proven that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Ten things are part of the fitrah: cutting the moustache, letting the beard grow, cleaning the teeth with a siwaak, rinsing the nose with water, cutting one’s nails, washing between the finger-joints, plucking the armpit hairs, shaving the pubic hair and cleaning one’s private parts with water.” (Reported by Ahmad, Muslim and the four authors of Sunan, from the hadeeth of ‘Aa’ishah, may Allaah be pleased with her). Even if the kaafiroon start letting their beards grow, this does not mean that the Muslims are allowed to shave theirs, because as we have stated above, the idea is not to be different from them in everything, only in the things in which they have deviated from the truth and from the fitrah.