to not punish someone for wrongdoing
—I know why that boy turned out bad. That’s what happens when you spare the rod.

One of Solomon’s proverbs is about raising children:

The one who spares his rod hates his child [He that spareth his rod hateth his son],
but the one who loves his child is diligent in disciplining him. (Proverbs 13:24)

Here, spare means “to not use,” and “rod” is a stick used for spanking, with the rod representing discipline.

A well-known phrase is similar to the first half of this verse, Spare the rod and spoil the child. It first appeared in the poem Hudibras, written by Samuel Butler in the 1600s.

Another well-known phrase that come from Proverbs is a gentle answer turns away wrath (or a soft answer turneth away wrath). It comes from Proverbs 15:1:

A gentle response turns away anger [A soft answer turneth away wrath],
but a harsh word stirs up wrath. (Proverbs 15:1)

And pride goeth before a fall (or pride goes before a fall) comes from Proverbs 16:18:

Pride goes before destruction,
and a haughty spirit before a fall. [Pride goeth before destruction, and an haughty spirit before a fall.]