The following is reported in a hadith narrated by Imam Bukhari (RA):

“Oppose the Mushrikeen! Lengthen the beards and shorten the moustaches. When Hazrat Ibn Umar (RA) used to go for Hajj or ‘Umrah, he would take hold of his beard in his fist. Whatever amount of the beard was in excess of the fist, he would trim it.” (Bukhari vol. 2, pg. 875)

In another hadith the following words are used:

“Make the moustaches extremely short and lengthen the beards.” (Bukhari Vol. 2, pg. 875)

It is wajib to keep a beard to the extent of one fist just as it is wajib to perform witr salah, and the two ‘eid salats. All the four imams agree on this issue. Allamah Shaami writes:

“To trim the beard when it is shorter than one fist in length as practiced by some people of the west and the hermaphrodites, has not been permitted by anyone i.e. any scholar.”

Hazrat Thanwi (RA) writes in Behishti Zewar (Heavenly Ornaments),

“It is haram to shave the beard as well as to trim it less than the length of one fist.” (Behishti Zewar section 11, pg. 115)

The word for beard in Arabic is lihyah which comes from the Arabic word for jaw. Therefore, the beard should be to the extent of one fist from the chin, the right side of the jaw, and the left side of the jaw, i.e. it should be the length of a fist from all three sides. Some people grow the beard to one fist from the chin but trim it to less than a fist from the sides. This is incorrect. If any one of the three sides of the beard has been shortened less than a fist, then this is a major sin.

Source:  The Four Actions

http://www.khanqah.org/books/show/waliullah-bananay-walay-4-amal