John Gill Commentary Genesis 21:26

"And Abimelech said, I know not who hath done this thing. Neither didst thou tell me, neither yet heard I of it, but to-day." — Genesis 21:26 (ASV)

And Abimelech said, I do not know who has done this thing: He pleads ignorance; he knew nothing of it before, nor now which of his servants had done it; intimating, that if he could know who it was, he should severely reprimand him for it.

neither did you tell me: signifying that he was to blame he did not complain of it sooner; and at least he had no reason to blame him, since he had never informed him before of it, and therefore could not expect to be redressed.

neither yet heard [of it] but today: he had not heard of it from others, as the Targum of Jonathan rightly adds, by way of explanation, but that very day, and very probably not till the moment he had it from Abraham himself.