The prophet who most eloquently described the wickedness of evil speech is the apostle James. His instructions are unequalled when it comes to mastery of the tongue.
'If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain.' (James 1:26)
'If any man offend not in word, the same is a perfect man, and able also to bridle the whole body.' (James 3:2)
'Even so the tongue is a little member, and boasteth great things. Behold, how great a matter a little fire kindleth!
And the tongue is a fire, a world of iniquity' (James 3:5-6)
'But the tongue can no man tame; it is an unruly evil, full of deadly poison.
Therewith bless we God, even the Father; and therewith curse we men, which are made after the similitude of God.
Out of the same mouth proceedeth blessing and cursing. My brethren, these things ought not so to be.
Doth a fountain send forth at the same place sweet water and bitter?' (James 3:8-11)
The Lord taught the equally sobering reality That every idle word that men shall speak, they shall give account thereof in the day of judgment (Matt 12:36). Here the Lord appears to be reading us our rights, for anything we say can and will be used against us in His courts of law.