Rashi on Deuteronomy 24:1/רש"י על דברים כ״ד:א׳

1כי מצא בה ערות דבר [WHEN A MAN HATH TAKEN A WIFE, AND MARRIED HER, AND IT COMES TO PASS THAT SHE FINDS NO FAVOUR IN HIS EYES.] BECAUSE HE HATH FOUND SOME SCANDALOUS THING IN HER; [THEN LET HIM WRITE A BILL OF DIVORCEMENT … AND SEND HER AWAY …] — it is his duty to divorce her because she should not find favour in his eyes (cf. Gittin 90b).