Question:
Is it permissible to clip the beard below a fist length? How long must a person grow his beard?
Answer:
It is haram to trim the beard below a fist length. Rather, this is worst than many other major sins due to it being a sin which is committed openly and will be regarded as a disrespect to Islam. It is also an open rebellion against Allah and His Messenger (صلى الله عليه وسلم).
(Ahsanul Fatawa 3/260)
It is wajib to let the bread grow a fist in length.
(Hazrat Mufti) Ahmed Khanpuri (May Allah preserve him)
(Mahmoodul Fataawa 4/724)