2 Peter 2:22

But it has happened to them according to the true proverb, The dog returned to his vomit, and the swine that was washed to wallowing in filth.

Matthew 12:45

Then it goes and takes with itself seven other spirits worse than itself, and they enter in and dwell there; and the last state of that man is worse than the first. So shall it be also with this evil 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 with itself seven other spirits worse than itself, and they enter in and dwell there; and the last state of that man is worse than the first. So shall it be also with this evil generation.
2 Peter 2:22
But it has happened to them according to the true proverb, The dog returned to his vomit, and the swine that was washed to wallowing in filth.