How should the Muslim trim his mustache? Is it permissible to shorten the beard? If this is permissible, what is the limit of this?

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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Ibn ‘Umar narrated that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Be unlike the polytheists, wear full beards and trim the mustaches."  [Al-Bukhari and Muslim]

Scholars provided several interpretations for the aforementioned command to trim the mustaches. Some of them are of the view that the hair which covers the lips has to be trimmed; others are of the view that it has to be shaved so that the whiteness of the skin underneath appears; a third group is of the view that it has to be completely shaved. Maalik however, considered the last opinion disliked under Sharee‘ah. The preponderant opinion is that it is better to adopt one of the first two options.

Shortening the beard is permissible for the one who performs Hajj or ‘Umrah to cut the hair beyond the fist of his hand. It was narrated that when Ibn ‘Umar performed Hajj or ‘Umrah, he used to hold his beard with his hand and cut what is beyond his fist. Otherwise, shortening the beard outside Hajj or ‘Umrah is against the command of the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, which signifies obligation.

Consequently, the person who shortens his beard, he actually does not wear a full beard and contradicts the command of the Prophet, sallallaahu ‘alayhi wa sallam. Allaah The Almighty Says (what means): {So let those beware who dissent from the Prophet's order, lest Fitnah strike them or a painful punishment.}[Quran 24: 63]

If the beard is extraordinarily long or wide in a way that distorts one's appearance, some of the scholars are of the view that it is permissible to shorten it in this case to become moderate and ordinary.