2 Peter 2:22

But the true proverb is verified in them, The dog hath returned to his own vomit; and the sow which was washed to its wallowing in the mire.

Matthew 12:45

Then goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. So shall it also be 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 goeth he, and taketh with himself seven other spirits more wicked than himself, and they enter in and dwell there: and the last state of that man is worse than the first. So shall it also be with this wicked generation.
2 Peter 2:22
But the true proverb is verified in them, The dog hath returned to his own vomit; and the sow which was washed to its wallowing in the mire.