He did not say, Stone her not, lest He should seem to speak contrary to the law. But God forbid that He should say, Stone her; for He came not to destroy that which He found, but to seek that which was lost. What then did He answer? He that is without sin among you, let him first cast a stone at her. This is the voice of justice. Let the sinner be punished, but not by sinners; the law carried into effect, but not by transgressors of the law.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Tract. xxxiii. 5