Matthew 12:35-37 KJV

35 A good man out of the good treasure of the heart bringeth forth good things: and an evil man out of the evil treasure bringeth forth evil things.
36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
37 For by thy words thou shalt be justified, and by thy words thou shalt be condemned.

Mark 11:23 KJV

23 For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.

James 3:2-5 KJV

2 For in many things we offend all. If any man offend not in word, the same is a perfect man, and able also to bridle the whole body.
3 Behold, we put bits in the horses' mouths, that they may obey us; and we turn about their whole body.
4 Behold also the ships, which though they be so great, and are driven of fierce winds, yet are they turned about with a very small helm, whithersoever the governor listeth.
5 Even so the tongue is a little member, and boasteth great things. Behold, how great a matter a little fire kindleth!