2 Peter 2:22

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

Matthew 12:45

Then it goeth and taketh with it seven other spirits more wicked than itself, and they enter in, and dwell there; and the last state of that man becometh worse than the first. So will it be also with 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 goeth and taketh with it seven other spirits more wicked than itself, and they enter in, and dwell there; and the last state of that man becometh worse than the first. So will it be also with this wicked generation.
2 Peter 2:22
It hath happened to them according to the true proverb, The dog returned to his own vomit; and, The sow that was washed, to her wallowing in the mire.