2 Kefa 2:21-22

For better it was for them not to have da'as of the Derech HaTzidkanut (the Way of Righteousness) than, having known it, to turn away from the mitzvah hakedosha handed down to them. The word of the true mashal (proverb) has overtaken them: KECHELEV SHAV AL KE'O (As a dog having returned to its own vomit), and a “chazir (swine) is washed only to wallow in the mud.”