Proverbs 26:11 in the King James Version
As a dog returneth to his vomit, so a fool returneth to his folly.
Context
10 As an archer who wounds all, so is he who hires a fool or he who hires those who pass by.
11 As a dog that returns to his vomit, so is a fool who repeats his folly.
12 Do you see a man wise in his own eyes? There is more hope for a fool than for him.