Leaving the beard to grow (upto a fist’s length*) and trimming the moustache is from the Wajib Sunnahs.
Al-Bukhaari, Muslim and others narrated that ‘Abd-Allaah ibn ‘Umar (may Allaah be pleased with him) said: the Messenger of Allaah (peace and blessings of Allaah be upon him) said:
“Trim the moustache and leave the beard.”
Narrated by al-Bukhaari, 5443; Muslim, 600.
According to another report:
‘Be different from the mushrikeen: cut the moustache and let the beard grow.”
Narrated by Muslim, 602.
And Muslim (383) narrated that Abu Hurayrah (may Allaah be pleased with him) said:
The Messenger of Allaah (peace and blessings of Allaah be upon him) said:
“Trim the moustache and let the beard grow; be different from the Magians (Zoroastrians).”
The Ahadith above have been taken from here.
Saheehus-Seerah 