Matthew 12:34-35

O ye generations of vipers, how are you, being evil, able to speak good things? for out of the abundance of the heart the mouth speaks.

Luke 6:45

The good man out of the good treasure of his heart brings forth the good; and the wicked man out of the wicked treasure of his heart brings forth that which is wicked: for out of the abundance of his heart his mouth speaks.

Matthew 15:19

For out of the heart proceed evil reasonings, murders, fornications, adulteries, thefts, false testimonies, blasphemies.

Mark 7:21-23

For within, out of the heart of men, proceed evil reasonings, adulteries, fornications, murders,

Mark 14:38

Watch and pray, that you may not enter into temptation: for the spirit indeed is willing, but the flesh is weak.

Hebrews 12:15

looking diligently lest any one may fail from the grace of God; lest any root of bitterness springing up may trouble you, and through it many may be defiled;

James 1:14-15

but each one is tempted by his own lusts, being drawn out, 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 you may not enter into temptation: for the spirit indeed is willing, but the flesh is weak.
Hebrews 12:15
looking diligently lest any one may fail from the grace of God; lest any root of bitterness springing up may trouble you, and through it many may be defiled;

For

Matthew 12:35
The good man, out of the good treasure (heart) brings forth good things: and the wicked man out of the wicked treasure (heart) brings forth wicked things.
Matthew 15:19
For out of the heart proceed evil reasonings, murders, fornications, adulteries, thefts, false testimonies, blasphemies.
Mark 7:21
For within, out of the heart of men, proceed evil reasonings, adulteries, fornications, murders,
James 1:14
but each one is tempted by his own lusts, being drawn out, and enticed.