C.32, q.6 a.c.1: "Ecce quod ista dimitti non valet cum innocens a fornicatione inveniatur. Sed ponatur quod ipsa sit adultera queritur si adulter possit dimittere eam causa fornicationis? Non enim videtur quod pari crimine infectus alium de eodem punire possit. Unde, cum Iudei adulteram accusarent, equitatis sententiam a domino acceperunt. 'Quicumque vestrum sine peccato est primus in illam lapidem mittat'."
Gratian: So, she cannot be dismissed since she is innocent of fornication. But let us suppose that she were an adulteress. Could an adulterer send her away because of her fornication?It appears that one guilty of a crime cannot punish another for the same crime. Hence, when the Jews accused the adulteress, they received a sentence of equity from the Lord [John. 8:7], ``Let him who is without sin among you be the first to cast a stone at her.'' The Gospel also commanded [cf. Lk. 6:42], ``You Hypocrite, first cast out the beam from your own eye, and then cast out the speck from your brother's eye.''