Sforno on Genesis 21:26/ספורנו על בראשית כ״א:כ״ו

1לא ידעתי מי עשה, even now that you have informed me by means of my servants that you have been unjustly victimised, I still do not know who are the ones who could have done this. Not a single one among the people concerned seems suspect to me. Had I suspected any one of my servants of such conduct he would not have been tolerated in my entourage.

2וגם אתה לא הגדת לי וגם אנכי לא שמעתי, what you have accused me of, that while in my position as king I had condoned violence. A king does interfere such matters only if one of two conditions exist 1) If the party that has suffered robbery complains loudly; 2) if neutral people aware of the robbery raise their voices in protest. In the case of Avraham, neither of such conditions existed until now.