Exodus 15:23-26

When they came to Marah, they could not drink its waters for they were bitter; therefore it was named Marah [bitterness]. The people murmured against Moses, saying, What shall we drink? And he cried to the Lord, and the Lord showed him a tree which he cast into the waters, and the waters were made sweet. There [the Lord] made for them a statute and an ordinance, and there He proved them, Saying, If you will diligently hearken to the voice of the Lord your God and will do what is right in His sight, and will listen to and obey His commandments and keep all His statutes, I will put none of the diseases upon you which I brought upon the Egyptians, for I am the Lord Who heals you.