Matthew 12:34-35

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

Luke 6:45

The good man out of the good treasure of his heart bringeth forth that which is good; and the evil man out of the evil treasure bringeth forth that which is evil; for out of the abundance of his heart his mouth speaketh.

Matthew 15:19

For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false testimony, blasphemies.

Mark 7:21-23

For from within, out of the heart of men, come forth evil thoughts, fornications,

Mark 14:38

Watch, and pray that ye enter not into temptation; the spirit indeed is willing, but the flesh is weak.

Hebrews 12:15

looking diligently, lest any one come short of the grace of God; lest any root of bitterness springing up trouble you, and the many be thereby defiled;

James 1:14-15

But each one is tempted when by his own lust he is led away and enticed;

Treasury of Scripture Knowledge

Summary

Keep thy heart with all diligence; for out of it are the issues of life.

Bible References

Keep

Mark 14:38
Watch, and pray that ye enter not into temptation; the spirit indeed is willing, but the flesh is weak.
Hebrews 12:15
looking diligently, lest any one come short of the grace of God; lest any root of bitterness springing up trouble you, and the many be thereby defiled;

For

Matthew 12:35
The good man, from his good treasure, bringeth out good things; and the evil man, from his evil treasure, bringeth out evil things.
Matthew 15:19
For out of the heart proceed evil thoughts, murders, adulteries, fornications, thefts, false testimony, blasphemies.
Mark 7:21
For from within, out of the heart of men, come forth evil thoughts, fornications,
James 1:14
But each one is tempted when by his own lust he is led away and enticed;