Trimming the Beard

Q: Is there any authentic narration which directly and emphatically mentions that trimming the beard is a punishable sin? How is sin and good determined in Islam? Is it Haraam or Makrooh to trim the beard? Did Nabi (Sallahu Alaihi Wasallam) ever trim His Blessed beard? If not, how is a fist lenght beard is an emphasised Sunnah? Which is more rewarding a trimmed fistful beard or a completely untrimmed natural beard? What is the ruling if one trims before it reaches a fist lenght? On what basis does a person continue to be sinful if he trims his beard before it is a fistful while trimmimg after a fistful is not sinful?