Sforno on Genesis 9:25/ספורנו על בראשית ט׳:כ״ה

1עבד עבדים יהיה לאחיו, his being in a position of dependence to his senior brothers, i.e. a plain עבד, was merely natural, and would not have been a special curse, especially in view of his disgraceful conduct. Solomon paraphrased this in Proverbs 11,29 when he spoke about ועבד אויל לחכם לב, that “a fool is a slave to the wise-hearted.”