Exodus 15:26

and said, If thou wilt diligently hearken to the voice of the Lord, thy God, and wilt do that which is right in His sight, and wilt give ear to His commandments, and keep all His statutes, I will put none of these diseases upon thee which I have brought upon the Egyptians; for I am the Lord that healeth thee. That was the test which the Lord proposed, namely, that the children of Israel should keep His commandments, laws, and ordinances. In that event He would prove Himself their true Physician in keeping from them the plagues which struck the Egyptians, and they could depend upon this promise as upon a definite ordinance.