1 Peter 3:15

but sanctify Christ as Lord in your hearts. And be ready always to give an answer to every one that asketh you a reason of the hope that is in you, but with meekness and fear;

Matthew 12:34

How can ye, evil as ye are, speak good things? For out of the abundance of the heart the mouth speaketh.

Titus 1:10-11

For there are many unruly vain talkers and deceivers, especially they of the circumcision;

James 3:6-8

And the tongue is a fire, a world of iniquity! The tongue among our members is that which defileth the whole body, and setteth on fire the wheel of life, and is itself set on fire by hell.

2 Peter 2:18

For speaking great swelling words of vanity, they allure in the lusts of the flesh, by dissolute ways, such as were in some measure escaping from those who live in error;

Treasury of Scripture Knowledge

Summary

The heart of the righteous studieth to answer: but the mouth of the wicked poureth out evil things.

Heart

General references

Bible References

Heart

1 Peter 3:15
but sanctify Christ as Lord in your hearts. And be ready always to give an answer to every one that asketh you a reason of the hope that is in you, but with meekness and fear;

General references

Matthew 15:18
But the things which proceed from the mouth come out of the heart; and these defile a man.