Matthew 12:34-35

Ye broods of vipers, how can ye speak good things, yourselves being wicked? for out of the abundance of the heart the mouth speaketh.

Luke 6:45

A good man, out of the good treasure of his heart, produceth what is good; and a wicked man, from the evil treasure in his heart, produceth that which is evil: for out of the abundance of the heart the mouth speaketh.

Matthew 15:19

for out of the heart proceed wicked reasonings, murders, adulteries, whoredoms, thefts, false testimonies, blasphemies:

Mark 7:21-23

For from within out of the heart of men proceed wicked reasonings, adulteries, whoredoms, murders,

Mark 14:38

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

Hebrews 12:15

carefully observing lest any of you fail of attaining the grace of God; lest any root of bitterness springing up trouble you, and by it many be defiled;

James 1:14-15

but every person is tempted, when by his own peculiar passion he is born away, and ensnared.

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 ye and pray, that ye enter not into temptation; the spirit indeed is willing, but the flesh is weak.
Hebrews 12:15
carefully observing lest any of you fail of attaining the grace of God; lest any root of bitterness springing up trouble you, and by it many be defiled;

For

Matthew 12:35
A good man out of the good treasure of his heart bringeth forth good things: and a wicked man out of the evil treasure of his heart bringeth forth evil things.
Matthew 15:19
for out of the heart proceed wicked reasonings, murders, adulteries, whoredoms, thefts, false testimonies, blasphemies:
Mark 7:21
For from within out of the heart of men proceed wicked reasonings, adulteries, whoredoms, murders,
James 1:14
but every person is tempted, when by his own peculiar passion he is born away, and ensnared.