וַיְמָאֵ֓ן ׀ וַיֹּ֙אמֶר֙ אֶל־אֵ֣שֶׁת אֲדֹנָ֔יו הֵ֣ן אֲדֹנִ֔י לֹא־יָדַ֥ע אִתִּ֖י מַה־בַּבָּ֑יִת וְכֹ֥ל אֲשֶׁר־יֶשׁ־ל֖וֹ נָתַ֥ן בְּיָדִֽי׃
But he refused, and said unto his master’s wife: ‘Behold, my master, having me, knoweth not what is in the house, and he hath put all that he hath into my hand;
Ramban on Genesis
BUT HE REFUSED, AND SAID UNTO HIS MASTER’s WIFE. Scripture relates that he refused to do her will even though she was his mistress, i.e., his master’s wife, and he feared her, for he feared G-d more. This is the meaning of the expression, unto his master’s wife.
Haamek Davar on Genesis
But he refused. The verse testifies that he, without any reasoning, refused to turn his cohabitation into fornication, but that he was forced to give his master's wife ulterior reasons apart from this.