Genesis 20:6 Commentary: Rashi, Or HaChaim, Radak & Siftei Chakhamim

'ידעתי כי בתם לבבך וגו I KNOW THAT IN THE INTEGRITY OF THY HEART etc. — It is true that at first you had no intention of sinning, but you cannot claim innocency of hands (Genesis Rabbah 52:6) —

ויאמר אליו האלוקים. G'd said to him (Avimelech). He agreed that Avimelech had not yet become guilty of death considering he had been convinced that Sarah was not married. As to his integrity, however, this was another matter. Whereas he might not have literally "robbed" Sarah [i.e. she did not belong to Abraham. Ed.], nonetheless he had employed the authority of being the king which is a kind of violence. At the very least "his hands were not clean in the matter."

ויאמר...בתם לבבךsignificantly, G’d did not add the words ובנקיון כפיך when acknowledging part of Avimelech’s complaint. The fact that Avimelech had not touched Sarah was evident, could be proved, whereas Avimelech’s intentions were private, could not be proved. Only G’d Himself would know if he spoke the truth.