Matthew 12:34-35

O degenerate race, how can ye, wicked as ye are, utter any thing that is good? for "'tis the overflowing of the heart that the mouth dischargeth."

Luke 6:45

a good man out of the good treasure of his heart produces what is good: and an evil man out of the evil treasure of his heart produces what is evil: for 'tis from the abundance of the heart, that the mouth speaketh.

Matthew 15:19

out of the heart proceed evil designs, murders, adulteries, fornications, thefts, false testimony, slanders.

Mark 7:21-23

for from within, from the heart of man proceed wicked designs, adulteries, fornications, murders; thefts,

Mark 14:38

watch and pray, that you may not yield to temptation: the mind indeed is vigorous, but the body is weak.

Hebrews 12:15

take care not to deprive yourselves of the divine favour: and that no poisonous root spring up, and spread a general infection among you.

James 1:14-15

a man is instigated, and drawn into the snare by his own vicious passions, which once indulg'd, are productive of sin,

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 yield to temptation: the mind indeed is vigorous, but the body is weak.
Hebrews 12:15
take care not to deprive yourselves of the divine favour: and that no poisonous root spring up, and spread a general infection among you.

For

Matthew 12:35
a good man out of the good treasure produces good things: and an evil man, out of the evil treasure, produces evil things.
Matthew 15:19
out of the heart proceed evil designs, murders, adulteries, fornications, thefts, false testimony, slanders.
Mark 7:21
for from within, from the heart of man proceed wicked designs, adulteries, fornications, murders; thefts,
James 1:14
a man is instigated, and drawn into the snare by his own vicious passions, which once indulg'd, are productive of sin,