2 Peter 2:22

It has happened to them according to the true proverb, "A dog having returned to his own vomit"; and "A sow that was washed, to her wallowing in mire."

Matthew 12:45

Then it goes and takes along with itself seven other spirits more evil than itself, and, entering in, they dwell there; and the last state of that man becomes worse than the first. So shall it be also to this wicked generation."

Treasury of Scripture Knowledge

Summary

As a dog returneth to his vomit, so a fool returneth to his folly.

Bible References

A dog

Matthew 12:45
Then it goes and takes along with itself seven other spirits more evil than itself, and, entering in, they dwell there; and the last state of that man becomes worse than the first. So shall it be also to this wicked generation."
2 Peter 2:22
It has happened to them according to the true proverb, "A dog having returned to his own vomit"; and "A sow that was washed, to her wallowing in mire."